I think the best way to measure this would be to think of it as a fraction of fuel consumption of a car going 30mph (city speed limit in NYC.)
The scientific way would be to express it as a volume measure, something like 10cc per minute or something (I’m making this up.)
What do you think?
Here is my guess:
A car idling will use 1/5th the amount of fuel used typical car is traveling @ 30mph.
What’s your guess?