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?