## The influence of speed on fuel consumption

This entry was posted by on Thursday, 27 February, 2014 at

Fuel prices soar, driving a car is just about as much fun as burning your money in the fireplace. Aerodynamic drag has a very large influence on how much fuel you use. Let’s have a closer look.

Aerodynamic drag increases exponentially as your speed increases:

Fd = 1/2 * rho * v² * Cd * A

That’s 1/2 times the density of the air, times speed squared, times the Drag coefficient of your vehicle, times the frontal surface.

The fuel that your car burns has to overcome this force. Some time ago the speed limit on some Dutch highways was raised from 120 to 130 km/h. To realise this ~8% in speed, you consume ~16% more fuel. In fact, the same law yields a ~30% reduction when driving 100 km/h.

Of course, besides the speed you could optimise the drag coefficient of your car by getting a more aerodynamic one, or the frontal area by getting a smaller one. But those only affect the outcome linearly, as opposed to the speed. Either way, trash that SUV because it is silly. (besides a high Drag coefficient and frontal area, it also has a high Coefficient of rolling resistance).

Don’t want to do it by hand? Use this handy tool at .

Today I had to make a 145 km trip of 95% highway, so I decided to let a big truck deal with my drag and drive behind it. Unfortunately my car has no fuelconsumption indicator, so I will only know the next time I refuel and calculate my average over the past tank.