[londons_explore]

If you really really care about miles per gallon, nearly any ICE car the most efficient way to drive is at or near idle in top gear, going about 25 mph. The difference is remarkable - a car which usually gets 50 mpg can easily get 100mpg with that approach, although obviously you can't practically drive like that on the public highway.

[hvidgaard]

It's not that simple unfortunately. It's most efficient when it runs the most lean fuel mix and least engine vacuum (i.e. full throttle). That balanced with gearbox efficiency, other drive train loses, and wind resistance. As a real world example, Ford Fiesta 1.0 EcoBoost is most efficient running 28mph in 4. gear. closely followed by going 44mph in 5. gear.

[WalterBright]

I thought it was 35.

But the most efficient way to drive a gas car is to speed up, then turn the engine off and coast down to a slower speed, then engine on for quickly moving up to a fast speed again.

Of course, the only way you'd want to drive like that is if you are alone in the desert and need to get to a gas station.

[gambiting]

I noticed that my plug-in hybrid Volvo does pretty much exactly this - whenever you let go of the accelerator, not only the car goes into neutral, the petrol motor does actually switch off entirely. When you want to accelerate again the electric motor kicks in first and then the petrol motor follows, so in practice if it wasn't for the small indicator on the screen you wouldn't ever know that it's happening. Considering the size and weight of the vehicle I'm actually seeing some incredible MPG when driven carefully.