In the EV space, cars like the Ioniq are way more efficient than most - the problem is that it doesn't matter much. The "fuel" is super cheap compared to petrol or diesel so no substantial saving. The benefits for range can be offset with bigger batteries - and the SUV/crossover shape (high riding but not a lot more ground clearance) gives that extra space to fit them.
As others say, so much work has already gone into improving aero efficiency with smooth undertrays and other details. Most mainstream cars have more efficient tyres than ever too. It's all been in the cause of dropping ICE car CO2 figures down a little to squeak in under a lower tax band but it means there's not much to gain unless you get really low or narrow (to cut down frontal area) and you can't do much of that without compromising on practicality.
Going forwards car designers need to properly rethink car design to better suit requirements rather than just emulating ‘conventional’ expectations of what a car should look like. Tesla is the biggest culprit with their front ends #gopping
what requirements? most teslas appear to be designed to keep the front as smooth as possible, whilst maintaining legal heights for lights etc. What else should be changed?
And because SUV’s are fashionable, bigger is better etc. Just look at any model car and compare 80s/90s size to now.
As pointed out, modern cars have to be bigger to accommodate all the mandatory crash protection, like airbags along the sides and for the passenger, instead of one for the driver in the steering wheel, greater crash protection in the doors, crumple zones, etc.
Look at the construction of an original Mini, compared to a base model BMW Mini, whatever that is, a Countryman is almost the size of a Berlingo!
But a lot of it is because people want SUVs “because I feel safer higher up”.
Citation needed, because I don’t believe it. I have a compact crossover, an EcoSport, which replaced the Octavia I drove for fifteen years, and I bought that after driving hundreds of different cars over roughly five years, and ‘feeling safer’ had nothing whatsoever to do with my decision; getting in and out more easily had everything to do with it. I really struggle to get out of BMW’s, often because the damn seats are set too low, and you have to get back out to allow the seat to go back to its full height before getting in again, but I was getting to the point with the Octavia where hauling myself out of it was becoming a chore, the Ford I more or less slide in and out, in fact I had a quick drive around our big storage area in a year old MX-5 recently, and I could get out of that easier than my Octavia, mainly due to the flaired rear arches letting me lever myself out with my upper arm. Getting my feet around the pedals with steelies on, however…
Anyway, I honestly believe it’s an aging population wanting vehicles that are comfortable and easy to get in and out of, while not needing loads of space for kids and stuff is what’s driving the sales.
If you need a more upright, natural seating position you use an MPV because that has the same floor height but much higher roof, and higher seats vs floor as a result. Rather than the whole thing just being on stilts.
Have you actually looked at how an MPV is put together? The seats aren’t much higher than a regular saloon/hatchback, but the roofline is higher, and they’re longer, in fact if you put a Zafira next to something like a Jag F-Pace, height is almost the same, but the Zaffy’s longer. And Berlingos and Partners are much bigger, but the driving position is truly horrible, I’ve driven enough of the bloody things multiple hundred-mile distances in exceptional discomfort to never want to drive one ever again, but they’re bigger than my Ford, which uses the Fiesta platform.
And the point you’re ignoring is the one I’ve just made above - MPV’s are designed for families and need to accommodate same, along with all the stuff that entails, SUV’s/crossovers are being bought by people who don’t have those needs and obligations.
I absolutely love my EcoSport, it ticks every box on my list of wants/needs in a car at my time of life, and I’ve only found one significant flaw - the reversing camera is too low, being just above the number plate where it picks up lots of spray; it should have been put up with the high-level brake light.
I’ve never bothered to check, but I do sometimes wonder just how many who criticise people who buy Kugas, EcoSports, Kadjars, etc, then get excited at the thought of owning a T4/5 or a Transit Custom, the seemingly essential lifestyle vehicle for a significant number of the STW membership…
"Cars have got massively more aerodynamic since the 80s. Smooth underbodies, hidden wipers, faired in headlights."
Far too many haven't. If anything it's gone in the opposite direction. Frankly, it's pathetic.
A Vauxhall Carlton saloon from 1986 had a Cd of 0.28 whilst the estate had a Cd 0.32. Both of these had vast interiors, huge boots, tons of legroom. Yet despite that the frontal area was tiny compared to so many cars nowadays.
The equivalent large family car nowadays is a some stupid (faux) off-roader with a worse Cd, disastrous frontal area and less room inside. And it weighs 50-100% more.
My Dad had a series of those Carltons as company cars. I remember it was easy to spot the estate one in a supermarket car park because it was taller than 99% of the cars in there, being a big estate with roof bars. Nowadays you'd never spot it amongst the fashion victim roly-poly/spine-destroying SUVs towering over it.
Modern engines and gearboxes are pretty amazing in their efficiency. It's a shame that allows them to be used in such inefficient overweight oversized vehicles and still get good performance and economy. It could be so much better...
SUV’s are not that less aerodynamically efficient than normal cars. They are higher up but they are have more ground clearance, so a bigger gap under the car and alot more air under the car so the frontal area is not actually that much greater than a similarly sized non-SUV.
The frontal area includes the area under the car. AFAIK, the CD is the proportion of the drag of the car to the drag of a flat plate of the same area. Even if an SUV has a decent CD, it still has a large frontal area and will be much draggier than a lower liftback type car.
A Vauxhall Carlton saloon from 1986 had a Cd of 0.28 whilst the estate had a Cd 0.32.
Audi and the Ford Sierra were pioneers in the low drag thing. After that, it quickly became standard. In 1980, CDs of .40 or higher were normal, by the 90s, below .30 became normal. That makes a huge difference if you are cruising on an autobahn because drag increases by the square of the speed (and power by the cube). At 60 mph, drag is four times the figure for 30 mph. Driving around town at low speed, with lots of stop-start driving, aero won't make much difference, but weight will. Cars now are much safer, but also much heavier. Hybrids with regenerative braking reduce the penalty of weight because they recycle some of the energy that would normally be dissipated by the brakes as heat.
I would argue that efficiency/mpg is easily beaten by not actually driving a car? During lockdown i have driven virtually no business miles yet we still do the same amount of business?
If i factor in trains, underground and the odd plane journey i didn't do then it becomes an impressive reduction.
16000 less miles
50 North East to London less Train rides
10 less intrnal flights
The above for my little business can be multiplied by 7 regular travellers.
Look at the construction of an original Mini, compared to a base model BMW Mini, whatever that is, a Countryman is almost the size of a Berlingo!
I read somewhere that the Mini Countryman is the same size as an Austin Maxi, which was a pretty big car in it's day (one for the older people!) and it still had a curb weight under 1,000kg.
But as already covered, I would rather crash in a new Mini than an Austin Maxi so getting weights back to 1970's weights is not going to happen.
Because they’re mostly sat in traffic at less than 12mph on a 2 mile return trip to Morrisons. So aerodynamics wouldn’t make a jot of difference.
+1. Came to say basically this.
A quick look at the M4 suggests there are plenty of people travelling further than that.
There may be. Yet...
More than half of car trips nationally are less than five miles. In urban areas such as inner London, a third of car trips are less than two miles
The combination of the pollution burst that is being created as cars warm up in the first five minutes of journeys, together with the large proportion of journeys being short ones, is making a significant contribution to the UK’s air pollution challenge.
Licence Bureau say the vast majority of journeys between 1 and 5 miles are covered via car.
SUV’s/crossovers are being bought by people who don’t have those needs and obligations.
Go past any school round here and the MPVs are significantly outnumbered by SUV and Crossover cars. Audi Q's Peugeot's with two 00's, Range Rover products, BMW X's, Kugas, Sportages etc.
Most two child families don't need an MPV for space. I don't need mine although it brings some significant advantages for carrying bikes and sailing gear. It suits us now with current lower mileages we are doing but it's a barn door aerodynamically. We only "needed" the space for camping gear but since we got a trailer it's mostly wasted.
If you didn't have kit heavy hobbies and only had two kids you'd easily get everything in a crossover or normal mid size estate (which I'll be going back to in a year or so).
Drac is kind of right about it comparing SUV platforms to older hatchbacks BUT the SUV is increasingly stepping into the ubiquitous family car/daily driver for the masses role and so in my view it's a valid reference if we define cars by use case not body styling.
Just look at all the Tesla’s that are being made. Not one of them is about being eco friendly with even there smallest car being way bigger than it needs to be
Maybe not more aerodynamic but they are making cars more efficient. My last 3 or 4 cars have all had better MPG than the one before it despite the cars getting bigger to fit a growing family.
On paper or in real use, Hooli? Is that the manufactureres figures or brim to brim and GPS distance?
As cars get more powerful/faster I've noted that if you use that performance the real world economy is dreadful anywhere except a motorway where you are at a constant speed and match the manufacturer's figures no trouble. It's only if you drive as if you were in the slowest (lowest power : weight ratio) car capable of being run through the test cycle that you'll achieve the manufacturers figures. Use the extra performance when pulling out of junctions etc. and the consumption will be lousy.
The test cycle flatters powerful cars.
Same with electrics, our new Zoé is less economical than the old one despite being only 20kg heavier. It's got a more powerful motor so my default style of using the power that's available in eco mode means I waste a bit more leccy out of every roundabout. It takes an effort to take things easy to achieve the figures we got with 20 bhp less.
If you drive faster and accelerate+ brake harder you will use more fuel/energy. You can't get away from that. Even with an ev regen isn't 100% efficient.
As well as all the other good points made above about space and usability, looks etc, there is another issue:
I studied a bit of aero as a hobby when I was into model planes and stuff, and important thing to remember is that below about 40mph it does not make a huge difference on a car.
what requirements? most teslas appear to be designed to keep the front as smooth as possible, whilst maintaining legal heights for lights etc. What else should be changed?
All of this....
It's almost like they said 'right, we don't need a large grille to cool the engine so let's just shrink it a bit. I have never liked Teslas because of the bonnets.
On paper or in real use, Hooli? Is that the manufactureres figures or brim to brim and GPS distance?
As cars get more powerful/faster I’ve noted that if you use that performance the real world economy is dreadful anywhere except a motorway where you are at a constant speed and match the manufacturer’s figures no trouble. It’s only if you drive as if you were in the slowest (lowest power : weight ratio) car capable of being run through the test cycle that you’ll achieve the manufacturers figures. Use the extra performance when pulling out of junctions etc. and the consumption will be lousy.
Real world figures rather than manufacturers. I've never really paid a lot of attention to the brochures MPG figures for exactly that reason.
and important thing to remember is that below about 40mph it does not make a huge difference on a car.
Small bet on all of those average journeys all being at sub 40mph, too.
The frontal area includes the area under the car. AFAIK, the CD is the proportion of the drag of the car to the drag of a flat plate of the same area. Even if an SUV has a decent CD, it still has a large frontal area and will be much draggier than a lower liftback type car.
Forntal area is a specific thing. The CofD is a coefficient that describes how aerodynamic the shape of the car is.
The frontal area describes the size of the thing - the hole it has to punch in the air. The frontal area of an aircraft for example isn't just the area of the machine plus the area of the 35,000 feet of altitude beneath it.
The C of D of an X3 is 0.29...pretty good and up there compared to any other car out there. The CofD of a 5 series is 0.28 so near as damn it the same despite the far more bulky and boxy appearance of the X3 - so hard to Judge and difficult to distill complex aerodynamics to one number. I'm struggling to find the numbers for frontal areas, but I'd hazard a guess the frontal area of the X3 and 5 series are similar (comparing those two as they are similar width). Though I'm sure ether 5 series is more aerodynamic overall it will be alot closer to an X3 than many would assume.
The wife's X3 is easily capable of achieving 50+ mpg if we were to drive it more sensibly. Right now its 40mpg but it does the bulk of short journeys so not the most efficient way of driving. And its permanent 4wd so alot of mechanical inertia to overcome compared to a 2wd car that saps away at efficiency. But on a long journey we're well into the 50's mpg and that's cruising at 'decent' motorway speeds. Could be more efficient if we cranked the cruise control back a bit.
The biggest influence on efficiency is the way you drive the car as Edukator describes. You can't have performance AND efficiency.
More than half of car trips nationally are less than five miles. In urban areas such as inner London, a third of car trips are less than two miles
Yeah of course but there are still plenty of people going further. The thing to remember is that the half of trips less than 5 miles is responsible for much less pollution overall because they are not going as far (even though the car is less efficient per mile).
Just look at all the Tesla’s that are being made. Not one of them is about being eco friendly with even there smallest car being way bigger than it needs to be
I don't think so - they need to be that big to fit all the batteries in - they don't make one with a 100 mile range do they?
In the EV space, cars like the Ioniq are way more efficient than most – the problem is that it doesn’t matter much.
I disagree, it's hugely important. The two main problems that people see with EVs are range and cost. The more efficient a car is, the greater the range will be for less battery and hence lower cost.
The cheaper Nissan Leaf has an on-paper range of 160 miles or thereabouts and has a similar sized battery (actually slightly larger) and cost about the same as a Hyundai Ioniq, but the Ioniq has 191 mile range. Anecdotally the gap increases further if you are driving faster e.g. on motorway trips.
The frontal area of an aircraft for example isn’t just the area of the machine plus the area of the 35,000 feet of altitude beneath it.
The aerodynamics of a car interact very strongly with the ground. Air doesn't flow freely under the car, you get a high-pressure region under the car and very turbulent flow.
The wife’s X3 is easily capable of achieving 50+ mpg if we were to drive it more sensibly.
People say 'ooh, 50mpg that's decent' but really, these days it's not. You can do much better than that in a similar sized car.
I'm looking at BMW's site, and they are quoting WTLP figures (as they have to) which are apparently much more representative of real world driving than NEDC was.
X3 (2.0d): 42-45 mpg
520d: 51-59 mpg
That's a dramatic difference. Now, you can say that manufacturers figures are bollocks, whatever, but as said this is the new cycle which is much better, and these figures are both from BMW and one would expect if they are fiddling it they would do the same for both cars, no?
I think it's easy to pick on a single element of car design, size, weight, height, aero or whatever and poke a finger as where these huge teams of talented people and billions of pounds of design went wrong, but you can't argue cars are more efficient now then ever, plus they're safer, more reliable, more recyclable etc.
As for the original question of more aerodynamic. Yes we can all remember the cars that out parents drove that were both huge on the inside and small on the outside but Governments, on our behalf, have decided that it's not acceptable to sell a car that won't protect all it occupants in a 50 mph accident, which means that a car from 2020 with the same interior space as one from the 80s will be much bigger. They could get around this 'easily' by say, using a lot of carbon fibre, which would also make them a lot lighter, but who wants to pay £70k for a Vauxhall Astra. Car manufacturers do work very hard on aero, they have to, emissions regs see to that.
Yes, the current trend for taller cars does make them less efficient, they're not really SUVs and they're certainly not 4x4s, they just have taller roofs so you can sit passengers a little higher and make it a bit shorter. For example the Skoda Kamiq (my Wife's car actually) has a efficiency rating of 45mpg combined, the Skoda Scala (hatch back version) is 46mpg.
Surely even just changing our typical UK/EU flat fronted design to the type you see in the USA more typically would be making a big difference.
#1. They have even worse visibility of their surroundings than euro style tracker units.
#2. Increases the train length.
A few people have mentioned slowing down. I made a reasonable comparison on a trip down the A1 from Durham to Leeds and back. Going I set the cruise control to 60 mph and returned 60 mpg. Coming home I set it to 70 mph and got 50 mpg. I was surprised at the difference.
My old R reg 1.9d Seat Ibiza used to return over 55mpg without any trouble.
Went on holiday with the GF up to Scotland (from Essex) sticking to A abs B roads. Remember calculating the mpg when we got home and it was over 60mpg. The car was well loaded, but speeds were never high.
Going I set the cruise control to 60 mph and returned 60 mpg. Coming home I set it to 70 mph and got 50 mpg.
Yeah but Durham to Leeds is downhill.
A few people have mentioned slowing down. I made a reasonable comparison on a trip down the A1 from Durham to Leeds and back. Going I set the cruise control to 60 mph and returned 60 mpg. Coming home I set it to 70 mph and got 50 mpg. I was surprised at the difference
Makes a huge difference in my Car Averaging 50mpg on A roads, I'll beat 50mpg easy, even with the roof racks on I'm too lazy to remove. Averaging 70 on the UK motorway and it's low-40s.
It's not the be all and end all though, driving down through France at 80, even with the car loaded up and bikes on the roof, I'll see just shy of 50mpg, the difference is France is a low flatter than Wales and the roads are quieter, you almost never have to brake / accelerate once you're up to speed.
I've mentioned this before, compare the Mokka-e to the Corsa-e
Both have a 45kWh usable battery, and 100kW power, 192 lbft torque.
Therefore imagining the exact same powertrain.
Weight unladen is 1530kg for the corsa, 1550kg for the mokka
Corsa is quicker to 60 too.
But the corsa has EVDB rated range of 170 and consumption 260 Wh/mi
the mokka 155 miles, 290 Wh/mi. (Theres no WLTP for the mokka on ev-database.uk)
Isn’t it the same with you create more efficient cars so now we can have bigger cars and that ‘slack’ created by the better aerodynamics is just taken up or cancelled out by the bigger cars.
Dunno what it's called, but it's certainly true. IIRC when you take an average the USA commutes twice as far as Europe, drives vehicles half as efficient, and pays 1/4 for the petrol. So the net financial cost is the same.
[strong]thols2[/strong] wrote:
QuoteThe frontal area includes the area under the car. AFAIK, the CD is the proportion of the drag of the car to the drag of a flat plate of the same area. Even if an SUV has a decent CD, it still has a large frontal area and will be much draggier than a lower liftback type car.
Neither of those things is true.
The area is just the frontal area (unless car manufacturers are fudging it in a way that no one else does?)
And Cd is derived thus:
It's still a bit of a fudge as it's dependant on Reynolds number, which will vary across the car (the Reynolds number of the wing mirror will be different to the Reynolds number of the bonnet).
I think your confusion that it relates to a flat plate is because it's dimensionless, so CdA is then expressed in m2 (and is generally <1). But a flat plate has a Cd, it's about 1.2 to 1.9 depeding on the Reynolds number.
The other bit you misrepresented the science on is "highly turbulent", flow is either laminar, transient, or turbulent. This is represented by the Reynolds number which is a way of expressing the ratio of inertial to viscous forces (velocity, density and a dimension divided by viscosity). It can be thought of as measuring the diameter of the smallest turbulent cell which can form in a system. So in a pipe if the dimension (the diameter) is smaller than the cell, the flow remains laminar. So on an object like a car which is big, traveling quickly, through a very inviscid fluid, the flow is almost always turbulent.
The diagram below shows how flow develops over a surface, note that as there isn't a constraint the flow always becomes turbulent as the turbulent cells can form.
This is actually a very good thing. Without turbulence you would end up with a huge area of negative pressure (it's that tumbling motion that allows it to rapidly change direction over the boot of the car) so you get a wake, rather than a stagnant vacuum.
But relating that back to drag coefficients, the amount of turbulence doesn't really matter. There's a Cd for an object within the laminar flow region, and a Cd in the turbulent flow region. It doesn't change further with more turbulence (once outside the transition region).
As a guide Reynolds number <10 is laminar, >10,000 it's fully turbulent. Typically for a car most of the components on it will have Re >100,000.
Isn’t it the same with you create more efficient cars so now we can have bigger cars and that ‘slack’ created by the better aerodynamics is just taken up or cancelled out by the bigger cars.
Not entirely. I learned to drive in a 950cc Fiesta Mk2 and it did about 35mpg, despite being visibly very flimsy and having no safety features beyond front seatbelts. First decent car I bought was a Seat Ibiza TDI which was far quicker, had way more tech, was infinitely better to drive, similar in size and could get 60mpg. The 2006 Prius we just replaced was way bigger than the Ibiza, similar MPG, much cleaner, packed with tech and safety features. A 2021 Prius is faster, bigger, more efficent and has more tech again.
Even an SUV like a VW Tiguan is much bigger, faster and more efficient than that Fiesta was. So yes, we're losing some efficiency gains by buying bigger vehicles, but not all of them.
I had a fluid mechanics lecturer in University who was very respected in the aero dynamic world. McLaren F1 team offered him a contract but he refused it, which I think he regretted. He kept the contract on the wall in class.
He asked us why we though cars weren't more aerodynamic especially compared to the 1950s. The sloped backs created a lot less drag and would make a massive difference in fuel efficiency... He believed the oil and gas industry had a huge influence on the decisions made to make cars less efficient.
Here is a nice old article about a manufacturer that paid attention to aerodynamics
http://www.citroenet.org.uk/miscellaneous/aero/aero01.html
There is another element of people are idiots to this.
Ask the general population what orientation of a "traditional teardrop" shape is more aerodynamic.
A soft nosed sportsback is about what we can expect so get out of a traditional "car" shape.
Pretty much a Tesla 3. (just why didn't they make it a hatch/5 door?)
I had a CX 🙂
Yep it doesn't matter how good the CD and engineering is if they can't sell them.
Pretty much a Tesla 3. (just why didn’t they make it a hatch/5 door?)
It would have broken the lines/look of the back window if they had a join.
I actually like the look of the model 3.
A soft nosed sportsback is about what we can expect so get out of a traditional “car” shape.
Not strictly true, the kammtail design (as used in a lot of modern bike frames, helmets etc) lets you use a conventional "blunt" end of an aerofoil shape for the front of the car, then that lip at the back creates an invisible tail of eddy currents which allow the air flowing off the body to stream off as if there was a full aerofoil.
It's the reason a lot of fairly blunt rear'ed cars aren't half as bad for fuel economy as your gut tells you they should be.
The Costin Amigo, reputedly the lowest production car CdA ever (the Cd low, and the A was tiny, just look at it!).
The answer to this question can be found in the Ford Kuga.
It is basically a Fiesta made heavier, less efficient, but BIGGER. And people (dare I say it especially yummy mummies) want a BIG car. So they can feel BIG and IMPORTANT.
Same with aerodynamics - if it doesn't make you feel like a Panzer commander it must be for people who earn less money you - or hubby.
Real world figures rather than manufacturers. I’ve never really paid a lot of attention to the brochures MPG figures for exactly that reason.
Unfortunately, too many manufacturers trying to game the system for too long. If you've ever driven an US product from the days of 55mph speed limits and CAFE, you would have experienced the wonderful feeling of a car designed to go 55. The transmission, lockup torque converter, engine, everything - all designed to to return max fuel economy at exactly 55. Drive at any other speed and it's much nicer but way less efficient.
To some degree the aero of a car is limited by crash safety and the like. You could have a far more aeor car that would separate a pedestrian at the knees in the event of a collision, the top half flowing gracefully over the car, the shins left in situ. It would not ever make it into the road these days though.
I’m hoping that we’ll see legislation to force this issue because I can’t see the the average British new car purchaser (us secondhand buyers are essentially irrelevant) moving away from this obsession with 4x4 style cars.
Driving less far is excellent. Driving slower likewise (I heard my last car going much more turbulent as it approached 80 and the fuel consumption dropped rapidly). But it all matters and if you drove less far, more slowly, in a smaller, lighter and more aerodynamic car with lower rolling resistance with an efficient powertrain then it would be better.
SUVs are fundamentally worse at everything, unless they’re a true 4x4 and you’re pulling horse boxes out of fields (so higher weight and traction matters). Worse aero. Worse handling and/or worse (high CoG requires stiffer suspension). Worse rolling resistance. More particulates from brakes and tyres. Less space for people or luggage from a given footprint.
It is basically a Fiesta made heavier,
it's nothing to do with the fiesta.
it’s nothing to do with the fiesta.
Yep, it's it's big brother the Focus.
The pointlessly jacked-up Fiesta is the bizarrely named and even weirder looking EcoSport, and slightly less weird looking but same concept, Puma.
Pedantry aside he has a point. People go out of their way to buy oversized cars.
I’m hoping that we’ll see legislation to force this issue because I can’t see the the average British new car purchaser (us secondhand buyers are essentially irrelevant) moving away from this obsession with 4×4 style cars.
We have (or we had). If no-one cared we'd still all be buying 35mpg cars. There was the 'agreement' with the EU and car manufacturers to reduce average emissions across the fleet to 120g/km CO2 or was it even lower? The problem is that for market reasons the average can still allow for less efficient SUV type cars.
You could say that every passenger car has to have emissions under 100g/km - but that's going to be moot soon because we'll all be buying electric.
You could say that every passenger car has to have emissions under 100g/km – but that’s going to be moot soon because we’ll all be buying electric.
True, although you could stipulate targets in miles/kWh or equivelent.
This gives me a great idea to solve the problem of boy racers locally. Now that all the cars will be controlled by a computer, make it so that they have to conform to the manufacturer's efficiency specs. If you want to get all pop and bangy in your stretched, cambered and fart-cannoned MX5 away from the lights. You have to earn it by driving like a nun all week for your Saturday night quarter-mile, otherwise, it cuts out halfway and your underage date goes away unimpressed.
I think there are 3 main issues. 1 weight cars are getting heavier which takes more energy to accelerate and decelerate. 2 the growth of suv and the war cars are getting bigger and bigger. 3.we drive a lot further whether it’s kids too school, the commute as well as for leisure.
specifically cars can get more aero. Many cars don’t need wheels and tyres as wide as they are, but they look good and sell cars. Rear wheels could be faired in like they are on many lorry trailers. I’m sure there are plenty of other things those who know about the science could do if it would sell
Way back when in the 80s Ford brought out the Sierra – possibly one of the first cars designed with aero in mind (dustbin wheel trims included)
TBH the Sierra had a pretty poor drag coefficient (Cd was about 0.37-0.4) and Cdx too, oh and that exciting design flaw that made it, ahem, "interesting" in sidewinds. I worked on a car by Daewoo (remember them?) where the Korean engineers decided to not do any aero testing and somehow managed to design the rear screen at the exact perfect angle that they got it to generate significant rear end lift when driving at high speed in nice constant gentle turns, like you get on say, a motorway...found out 4 weeks before production start! My old design boss at Ford would tell stories of how the Audi A2 aero was dramatically improved with two drinking straws attached to the rear lamps...But I digress...
Aerodynamics is a still a bit of a black art (spent more than a few interesting, very long late night sessions in wind tunnels or reviewing/correlating CFD data playing about with various cars while have aero engineers drumming this into me) and is just part of the overall efficency of the vehicle. It needs to be done with respect to a lot of other factors, sidewind/crosswind stability is very important, making sure you don't create lift or indeed downforce as they can both muck up the cars dynamics if you get things wrong, getting cooling air into and then heat out of engines, rolling resistance (tyres have a huge impact on mpg), mechanical resistance, weight (screwed up by those pesky crash tests...) speed (very rough rule of thumb is you use 25% more energy to go at 80mph than 70mph) etc etc. Oh, and not driving with a lead right foot helps a great deal too...
