[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.