>Rough metric is you get 1 hour of engine idle per gallon. Some cars do better than that.

I have a non-plug in hybrid, that gets almost 50 mpg consistently; I've got to experiment and extrapolate to see if it's much better. I would like to believe so.

I recently learned of the existence of:

https://en.wikipedia.org/wiki/Gasoline_heater

and wonder how efficient they are.

These seem to be mostly outdated, but I guess that they are particularly useful with air cooled engines, where a conventional heating system doesn't have coolant to work with.