Why does driving at 70mph in still air use more fuel than driving at 60mph with a 20mph headwind?

Question

Surely with a 20mph headwind @60mph the 10mph greater aerodynamic drag will outweigh the extra revs friction @70mph.
Would the fuel consumption be related to the road speed + relative wind speed?
So driving at 50mph with a 50mph headwind would give only slightly better consumption than driving at 100mph on a calm day?

The only reason driving faster uses more fuel is the extra aero. drag. In fact each HP is produced more efficiently the faster the engine runs upto a point.
Surely 70 = 60+10wind so they would have the same energy requirement but it doesn't seem the case, why?

Many website say driving 10mph over 60mph increases fuel use by 15%. Yet a 10mph headwind only increases fuel use by 5%. Why?

Answer 1

There are 2 factors at work here:
[1] aerodynamic resistance,the drag caused by wind resistance
[2] rolling resistance,the drag caused by friction in moving parts.
Each one increases on a logarithmic scale with speed.

Answer 2

You do the maths:-
rolling resistance and mechanical efficiency are for all practical purposes directly proportional to the speed ie a straight line on a graph.
Wind resistance increases by the cube of the speed difference ie to double the speed you need 8 times the power (This fact makes a mockery of some performance claims)
Therefore, a car with exceptionally low drag but with under inflated tyres and slightly binding brakes will use less fuel @ 60 + 20 headwind (80 airspeed) than at@ 70 with no wind

Answer 3

I doubt very much that this is in fact the case. Fuel usage is dependent on load and the vast majority of the load at high speeds is the air drag, therefore fuel usage rate will be higher at 60 mph into a 20 mph wind, than at 70 mph. Note that the drag increases by at least the square of the apparent air speed. Additionally if you look at the usage for a particular trip, rather than the usage rate by time, at 70 mph the trip will be shorter duration, thus usage will be less again, due to the shorter time. One additional factor is that the engine might operate a little less efficiently at the revs required to go 70 mph as opposed to the revs required for 60 mph but this will be dependent on the characteristics of the engine and the car's gearing and it is very unlikely to override the other factors. In short, I do not believe the premise as stated.

Answer 4

Its simply because the friction created by the road is greater then the friction created by the wind.

Just think about it. If you drive along at a steady speed and then take your foot off the accelerator and cruise to a halt then it won't take very long. A head wind will make very little difference to the distance you would roll.

To put that anaolgy into your question, it simply because the road is grabbing the car more than the wind is. So the faster you go, the more power you have to use to overcome the road friction. You do not need much power to overcome air friction.

To get the power needed, you have to put the boot down and therefore the car drinks more petrol.

Answer 5

To drive fast is dangerous, if you are driving at 80 or 90 miles an hour and you blow one of your tyres, I guarantee you that it would be very difficult to control that car with disastrous consequences. Now when we are talking by law, well you are breaking the law it doesn't matter if the road is clear, the police can't stop all the drivers driving at more than the speed limit, it would be impossible but they can do it if they want it, they use another method like speed cameras, sometimes you don't see them but they are there, so many of those drivers breaking the law I assure you that they will receive a letter through the post. The tolerance is 10 percent, it means if the speed limit is 70 miles you will be tolerated to drive up to 77 miles/hour, so be careful, not only by law but also for your own safety. You must drive the car in the manner that you can control it when something unexpected happens. Remember that there are a lot of people in the motorway or roads if it is that the case. Good luck.

Answer 6

not sure, but i would guess the engine's efficiency decreases once you pass the peak of your torque curve.

some interesting physics reading here (if you'd like to learn more):
http://craig.backfire.ca/pages/autos/horsepower

good luck & have fun! :)

PS - how did this question come up? is this a theoretical question or based on observations? do you have repeatable measurements showing your fuel efficiency is in fact increasing in the 80 mph windspeed scenario?


@Tridawgn -- if the definiton of "perfectly aerodynamic" = zero drag, then the only thing that is perfectly aerodynamic would be an object with zero surface area. _any_ finite surface area presents an opportunity for drag.

Answer 7

Most cars are designed to give best economy at below 60mph. the more you increase the speed, the further you get away from the economical area. Aerodynamic drag is just one of the factors involved.

Answer 8

I guess it depends on the aerodynamics of your car.. if your car is perfectly aerodynamic the wind wouldnt factor in at all, but then again if your driving a semi.. it would affect it alot more.

Answer 9

More power used in engine friction, transmission drag, tyre drag &c.

Answer 10

It just makes sense that driver faster would take more fuel than driving slower. Think about it.