Morning all!

It is easy to find the MPG of vehicles for “urban” (~30 mph) and “motorway” (~70 mph) driving, but what about if a car needs to be driven really slowly like ~5-25 mph for extended periods – like when following a bunch of cyclists riding in the mountains?

I have been asked to be a support driver for a group and I will be taking out a Citroen Berlingo for two ~8 hour days and I want to get an idea of what the fuel costs will be to ensure that I/we charge them correctly for this service so that it covers both the fuel and my time.

Better or worse than “urban” would be a start.

Any help would be gratefully appreciated.

surely you don’t actually crawl along behind them for the whole day?

urbans more “stop start ” driving as oppose around 30mph driving

if you can crawl along at 5mph all day at idle in first – youll be impressed with the mpg 😉

the more changes in speed the more fuel you will use.

We get roughly our max mpg (50+) when in 4th at 25-30mpg with a very light foot, but any variation in speed and inclines makes it suffer.
I would base calculations on what you get ‘around town’.

Ours is a 2011 1.6d (90ps) Peugeot Partner (same as a berlingo), and we get about 38mpg around town when not focussing on mpg.

Hey Jam bo,

I am there to carry clothes, food, bottles and spares and so I will need to be close by *most* of the time. It is a bit like a team car in a race – riders will want to drop off and collect stuff as they ride.

45ppm and then for your time.

trail_rat and AlexSimon – thank you.

It will have some reasonable inclines – we are out in Southern Spain at the moment. Usually, I am a riding guide but this group want a car instead in case any of them start to flag and need a lift up a big climb or even all the way home. The rides will be ~140-160 km and 4-5000 m of climbing and these guys (and girls) are not pros, hence the need for a “safety net”.

Not exactly what you’re after but should give you an idea

Pretty much how smoothly you drive has more of an impact than the speed you’re going at.

b r – thank you. They have offered 160 euros per day, which kinda fits as we charge 100 euros per day as “bike guides” so that rate is pretty similar. But, I don’t get to ride and work on my tan lines.

Scaled – thank you!

Trying to drive smoothly will be a nice little challenge and will keep me occupied during the day.

That’s cool – I will confirm everything with them and get to work on my left arm tan/burn lines. Thanks everyone, much appreciated.

youll be impressed with the mpg

Unless you’re being ironic I doubt that muchly – all engines have a sweet spot where they have maximum fuel efficiency.

Sitting at idle or at low revs uses significantly more fuel than driving at 56mph in top gear.

Add in the inclines and you’ll be watching the fuel gauge drop faster than a thing dropping quickly…

fit some bullbars and “motivate” them

somebody didnt read the link did they .

1.6 citroen diesel – at 20mph did 99mpg at 90 did 27mpg….

“Tests on five different cars ranging in size from a 1 litre Toyota Aygo to a 2.2 litre Land Rover Freelander found that the most efficient speed was below 40 mph for all five and as low as 20 mph for two.”

johnellison – Member
youll be impressed with the mpg
Unless you’re being ironic I doubt that muchly – all engines have a sweet spot where they have maximum fuel efficiency.

Sitting at idle or at low revs uses significantly more fuel than driving at 56mph in top gear.

Physics isn’t a strong point of yours is it…

Unless you’re being ironic I doubt that muchly – all engines have a sweet spot where they have maximum fuel efficiency.

I’d always understood that because wind resistance increases exponentially with speed rather than linearly, you will always use less fuel for a given distance the slower you go? It’s the same logic for why, when on a bike, and you ride an out and back course with a head wind then a tail wind, you never recover all the speed you lost on the headwind leg, on the tailwind leg.

Sitting at idle or at low revs uses significantly more fuel than driving at 56mph in top gear.

Yes, but in the scenario the OP suggests, he won’t be at idle or low revs, the selection of an appropriate gear will ensure the engine is still running in that ‘sweet spot’ interms of revs/torque, even though the speed is low.

That’s my understanding anyway, but willing to be corrected by someone who understands the fizziks better than me 🙂

I’ve always thought about this. When I drive my mums Touran 2L diesel I get better MPG when doing 35mpg than 25mph on a 10 mile trip (average). I always try and get the highest MPG so no bias on speed. Using the gears I usual sit at 1800 rpm. I average between 58-63mpg @35mph vs. 54-58mpg @25mph. This is on country roads that are flat-ish. It baffles me.

So wind resistance doesn’t play any part whatsoever in fuel efficiency – there’s no cubic function involved at all….

I’ve always thought about this. When I drive my mums Touran 2L diesel I get better MPG when doing 35mpg than 25mph on a 10 mile trip (average). I always try and get the highest MPG so no bias on speed. Using the gears I usual sit at 1800 rpm. I average between 58-63mpg @35mph vs. 54-58mpg @25mph. This is on country roads that are flat-ish. It baffles me.

What baffles me is why anyone would regularly drive at 25mph on a country road, except for the old gimmers on their way to collect their pensions…

b r- firstly the speeds are average. Secondly I’d like to see you go any faster than 40mph on the roads I drive. Factor in a few villages.

if they are average speeds then at 35 you probably just stopped and started less.

Sitting at idle or at low revs uses significantly more fuel than driving at 56mph in top gear.
and here was me thinking that putting my foot down added fuel to the engine to get more power – what happens then ?

You are confusing efficiency and economy – ie its maximum torque or how much of the energy it uses from the fuel with how much fuel it uses

A mate genuinely thought that despite doing fewer mpg at 70 it used less fuel as your drove for less time – it took hom ages to be perusaded and get that there was no time element to mpg and i had to write it all down

It’s the same logic for why, when on a bike, and you ride an out and back course with a head wind then a tail wind, you never recover all the speed you lost on the headwind leg, on the tailwind leg.

Wind resistance isn’t the whole story though – the other factor involved is that because you’re going slower into a head wind, you’re being slowed down for longer on the outbound leg than you are being speeded up on the inbound leg. Although you can make up time on the inbound leg, you have less time to do it, so can never get the average back to the point where it would be if both directions had no wind.

As far as MPG is concerned, speed and smoothness definitely have a part to play. Just from gut feeling, smoothness seems to be the bigger contributor.

In my car i get 15-25 mpg in first or second gear between 5-20 mph. At 70 i get 40-45 mpg. I think there must be a lot more friction in the lower gears.

could also be that alot of the energy expended when most folk do 15-25mph is used accelerating through the range

as oppose to when at 70 your maintaining a constant speed.

Getting low mpg at very low speeds will be because you are stopping/starting/braking/using a low gear. Get the car lugging along at tickover-ish and minimise acceleration and braking and you will get far better economy.

Due to resistance (air and road) it always requires more energy to maintain a higher speed.

The problem with applying this to respective fuel usage is introduced by engine efficiency and gearing.

In my car i get 15-25 mpg in first or second gear between 5-20 mph. At 70 i get 40-45 mpg. I think there must be a lot more friction in the lower gears

You can do 70mph in first gear?

Why would you drive along in 2nd at 15-25 mph? All cars will happily lug along at tickover – all the driver has to do is keep a nice big gap (multiple seconds) to the traffic ahead and use the gap to stop all the micro accelerating and braking.

The sweet spot for most cars for maximum mpg is around 56 mph. This is because above that speed the exponential rise in aerodynamic drag (drag does increase at the square of speed) becomes the overwhelmingly dominant force to overcome. This should be a familiar concept to all cyclists, especially if you’ve drafted a car, ridden into a headwind, or just lifted your body up on a fast descent.

Traveling slower means your rate of covering ground is lower and the amount of fuel required to just keep the engine running (to overcome the mechanical friction, inertia of rotating masses, pumping fluids round the engine, driving power steering and air con etc) becomes a greater proportion of fuel used relative to the proportion of fuel used to actually generate forward motion (the best you can possibly expect is 45% efficiency – machines are not very efficient at all). So proportionately less fuel is being used to generate forward motion. Also if you’ve got a turbo charged car, traveling slowly usually means you’re off boost which significantly reduces your engines efficiency. Your on-board mpg computer is not particularly accurate. I have no idea what the mpg would be traveling at 20mph vs. 30, I’d guess it isn’t that different. Urban mpg figures may take into account stop start conditions, but the acceleration will be low, and you save some fuel on the stop part, so I don’t think will be too detrimental compared with a steady 20mpg. It will most likely be influenced more by the individual cars gearing and the drivers driving style.

bails – Member

You can do 70mph in first gear?

No smartarse 😉 but I can just about do it in second if I let it buzz the limiter 8)

glenp – Member

Why would you drive along in 2nd at 15-25 mph? All cars will happily lug along at tickover – all the driver has to do is keep a nice big gap (multiple seconds) to the traffic ahead and use the gap to stop all the micro accelerating and braking.

Depends on the gearing, on my car third gear is labouring at 20. Also it’s not a great idea driving at low revs, especially in a diesel as it can cause wear to the DMF.

DMF? What’s that?

The engine will only labour if you’re trying to accelerate or pulling up a hill – just rolling along as if in snow, letting it lug along at tickover or a bit more, is no problem. I think the problem is that people want a sudden burst of acceleration all the time, rather than just rolling with it.

glenp – Member

DMF? What’s that?

The engine will only labour if you’re trying to accelerate or pulling up a hill – just rolling along as if in snow, letting it lug along at tickover or a bit more, is no problem. I think the problem is that people want a sudden burst of acceleration all the time, rather than just rolling with it.

Dual mass flywheel.
Also I don’t think the vibration would do the engine mounts any good.

DMF= dual mass flywheel?

When do you ever drive for a long period of time at the same speed between 5 and 20mph? Mpg is probably so low at that speed because you’re constantly on and off the accelerator and the brakes. If you went to a flat track and put the car in 2nd at 15mph/3rd at 20 and just drove and drove and drove then you’d get pretty decent mpg out of it.

Not convinced. Vibration or shudder is from asking for more from the “wrong” revs – there is no need for that. 99% of the time just rolling with almost no power, then change gear if you want to accelerate. Driving relaxed and slow in a high gear doesn’t create any problem, provided you use common sense and select a suitable gear to accelerate with.

Some people just don’t want better economy I guess. Or rather, they aren’t prepared to make any change to their habits.

Edit – I frequently drive at 20mph – through towns and high streets. If you avoid the silly temptation to mirror exactly the speed of other cars (accelerating/braking all the time) it is easy and relaxing and economical.

glenp – Member

Not convinced. Vibration or shudder is from asking for more from the “wrong” revs – there is no need for that. 99% of the time just rolling with almost no power, then change gear if you want to accelerate. Driving relaxed and slow in a high gear doesn’t create any problem, provided you use common sense and select a suitable gear to accelerate with.

But the OP is driving in mountains..?

My own figures come from driving every day on a flat road with a 20 limit.

Some people just don’t want better economy I guess. Or rather, they aren’t prepared to make any change to their habits.

That’s nonsense. I want the best economy possible, just not at the expense of wearing other expensive parts out.