What’s the most efficient speed to drive a diesel car on the motorway? What are the influencing factors if there’s no easy answer?

Is that efficient in time/money or mpg/money?

And if you’ve the time, probably 45mph.

I think this has been done many times before.

In my car I only get a couple of mpg difference between 70mph & 85mph so I just go the speed I want. I think if you drop your speed even more though the mpg does go up, but then I think its dangerous and stupid to do less than 70mph on a motorway.

surely it depends on your torque curve, gear ratios, where there turbo kicks in etc etc. Probably the speed at which the car still feels comfortable on a soft throttle in top gear

Slower = more efficient, as long as you’re in top gear and the engines going fast enough to run smoothly it’s probably at it’s most efficient. So sit in the slow lane with the trucks at 56mph (anythign slowers anti social) for best efficiency.

FWIW I get 40mpg from my petrol C-max doing a steady* 70mph on the motorway, which I thought was shockingly bad, but then similar diesels only seem to do 48 so I lost the urge to replace it despite doing about 1800miles a month.

*this is the important bit, if I swing between 65 and 75 (so the same average) the mpg plummets as it’s having to accelerate all that weight.

surely it depends on your torque curve, gear ratios, where there turbo kicks in etc etc. Probably the speed at which the car still feels comfortable on a soft throttle in top gear

Think of it as a ballance, energy in = energy out (unless your accelerating or decelerting).

Slower means slower engine speeds (less friction) less aerodynamic drag, less everything. So energy out is minimised, energy input is minimised. The engine management then knows the most efficient ammount of fuel, advance/retard etc for that energy requirement.

“anythign slowers anti social”

What?? Surely driving slower than that is dangerous !

“anythign slowers anti social”

What?? Surely driving slower than that is dangerous !

I got flamed for suggesting bikes shouldn’t be allowed on dual carrigeways where they’re essentialy motorways as apparently it’s the car drivers driving allong assuming everything else is doing 56-70mph that are in the wrong, not the preson going 25mph.

At a steady 60mph my car achieves approx. 50mpg, at 70 45mpg and 80 about 37mpg (this is from the fuel computer which is reasonably accurate on a longer run), which is what you would expect as the aerodynamic drag increases with the square of the speed. This is why when I go to North West Scotland we stick to a steady 70 on cruise as in the 600 miles we save 20% on fuel compared to going at 80mph but it does take an hour longer. With fuel at is current price we think that is a worthwhile trade

Someone recently brought this car to my attention: Alpina D5

No car that develops 345bph and 516lb/ft has any right returning 47mpg and 155g/km CO2.

It’s ridiculous. It’s a freakin rocket ship that is more efficient and less polluting than my sub two litre diesel C-Max!

I think its dangerous and stupid to do less than 70mph on a motorway.

What !?

Are you serious.

In my diesel panda I get (ish) 60mpg at 70mph, 55mpg at 80mph and 80+mpg at 56mph according to the fuel comp,

jimw – Member

At a steady 60mph my car achieves approx. 50mpg, at 70 45mpg and 80 about 37mpg (this is from the fuel computer which is reasonably accurate on a longer run), which is what you would expect as the aerodynamic drag increases with the square of the speed. This is why when I go to North West Scotland we stick to a steady 70 on cruise as in the 600 miles we save 20% on fuel compared to going at 80mph but it does take an hour longer. With fuel at is current price we think that is a worthwhile trade

Thats quite a drop of mpg between 70mph & 80mph

With my 6 speed gearbox & at 60mph my comp. claims I get better mpg in 5th compared to 6th?

(diesel golf)

But it flips when you get to 70mph?

In both my cars I get about 10% less at 80mph than 70mph. Much less air resistance. So the slower the better as long as you are still in top gear and above about ooh.. 1300rpm ish I suppose.

However in certain cars – very large petrol engines that are not direct injection – you can sometimes get better economy by driving a bit faster because the amount they have to be throttled back dramatically reduces efficiency which more than cancels out any gains made by slower speeds reducing air resistance.

With my 6 speed gearbox & at 60mph my comp. claims I get better mpg in 5th compared to 6th?

If revs are too low then to develop the same power (and hence drive at the same speed) you put a little more fuel in each cylinder but of course there are fewer cumbustion events. So there is a larger burn, and the gas stays in the cylinder longer. Therefore the gas is more incontact with the insides of the cylinder and piston and so more heat is lost to the cooling system.. Resulting in less efficiency.

Also see above re large petrol engines – throttling back reduces efficieny in petrol.

The drop is probably due to quite a large frontal area and a bigish engine and possibly the four wheel drive transmission

Edit:

another thing that makes a big difference in my cars consumption is tyre pressure. If I pump the tyres up from the stated 38 front 34 rear to 42 front 38 rear the consumption drops by about 10% on a longer run, but it does make the ride less comfortable

Aerodynamic effects take over at between about 30 to 35mph. So below that weight of the car is the predominant factor.

2 factors influence the aerodynamics, frontal area and drag coefficient.
Drag inreases with the square of speed. 70 to 80 is generally accepted to be associated with a 15 to 20% decrease in consumption. If you have a high frontal area (mini MPV / SUV) it will be worse than a slippy saloon.

But this is all steady state. what influences consumption more is accelerating. How you drive is most important. Commuting at 80 on an empty motorway will produce better figures that 80 on busy commuter time motorway.

My best consumption figures are produced on cruise control. Using this it is simply staggering how much everyone else is varying their speed.