# How can a petrol car require less fuel at 55mph than a lower speed at the same rpm?

It seems to be widely accepted that somewhere around 55mph is the most economical speed to drive in a conventional petrol car.

Recently I ordered an EV, and learned that (with the exception of at very low speeds), efficiency decreased as speed increased. As I thought about this, it made perfect sense - as air resistance increases, so does energy consumed.

However that's left me unable to explain this 55mph thing...

Let's assume my petrol car does 55mph at around 2,500rpm in top gear. Let's also assume in second gear, 2,500rpm is about 20mph.

How can it require less petrol to run the engine at 2,500rpm pushing a heavy car at 55mph than the exact same engine doing the same rpm only pushing at 20mph (far less air resistance)? Surely it has to need less petrol to move at 20mph if the engine is running at the same speed?

The only possible thing I can think; is non-engine uses of the fuel (AC/stereo/etc.). Since they don't change with speed, the faster you go, the less fuel they're using per-mile. However for this to work out, it seems like they'd have to make up a significant portion of use of fuel; which is very unlikely?

• The combustion engine efficiency varies in both torque and power depending on RPM and the torque dependency is different from the power dependency. If a gear ratio is chosen to give the most efficient engine RPM at 55mph, it will be more efficient than going slower or faster within some boundaries, like maybe 50 to any v>55mph. If the gear ratios are good, I can't imagine 55mph being better than 30 or 35mph. However, going 30 on the freeway not a good idea. I would guess a little research will show the efficiency measurement is constrained to highway speeds. Commented Mar 20, 2015 at 22:43
• Graph of efficiency vs speed for a few cars. ecomodder.com/forum/… Commented Mar 20, 2015 at 23:29
• It's not that efficiency is optimal at 55 mph. It's just that the "knee" of the curve is around there, and it's hard to ask people to go much slower on the open highway. Generally "optimal" is going to be around 35 mph or so. Commented Mar 20, 2015 at 23:34
• Surely cruising at 2k rpm at 55mph should still require more power-per-mile than cruising at 2k rpm at 20mph because of air resistance? The torque should be the same at the same rpm? What's causing it to be so inefficient at low speeds (which doesn't seem to affect EVs the same? Commented Mar 21, 2015 at 8:08
• Just drive the car in first gear for an hour, it doesn't need much gas, but it doesn't go many miles either, so the miles per gallon is low Commented Mar 21, 2015 at 14:16

How can it require less petrol to run the engine at 2,500rpm pushing a heavy car at 55mph than the exact same engine doing the same rpm only pushing at 20mph (far less air resistance)?

It doesn't require less petrol, it requires more.

However, if it requires less than $\frac{55}{20}$ times more petrol at 55mph, then the car is more efficient (greater mpg) at 55mph than at 20mph.

• Sorry, this may be worded badly; I meant "less petrol per mile" not per revolution. What causes the petrol engine when driving at lower speed to do less mpg when the air resistance is so much lower? Commented Mar 21, 2015 at 8:05
• @DannyTuppeny, assuming it is the case that it is more efficient to 55 than at 20, it might be that the air-resistance is insignificant compared to the engine and drive-train losses at these speeds with air resistance becoming significant at higher speeds. Commented Mar 21, 2015 at 11:33

The power required to push the car forward increases with the cube of speed. Your car's engine has been designed to deliver a certain power, and asking less from it will make it run at a lower efficiency.

You still need to move all those bearings and turn the coolant pump and the generator, so a certain base load is always there. Driving in a lower gear, but at the same engine speed, means that the useful power which the engine delivers to the wheels will be lower in comparison to this base load. This is one source of inefficiency.

Engines with a catalytic converter need to run at a fixed fuel-to-air mixture, and doing the same RPM at low speed means you still need to feed your engine the same amount of fuel, but will extract less useful power from it. This is a second source of inefficiency.

Driving at 20 instead of 55 mph translates into into a 2.75 times higher operating time to cover the same distance. Obviously, keeping the engine running for a longer time will increase fuel consumption proportionally. This is a third source of inefficiency.

because higher speed pulls more oxygen to burn into the engine and less fuel is needed.