Is there a decent translation for Engine Efficieny to mpg, Which I know depends on a number of factors. But Id like to know if my engine is getting the equivalent of 3 mpg…
It’s not really possible to convert to MPG, but I can tell you that really ineffecient engines might be as bad as 20%, really good ones up to about 35%.
Ok thanks for the reply! I just needed to know an example. I just had nothing to base it off of.
Engine efficiency is basically the inverse of the g/kWh measure, while a bit easier to understand as a convention. 35% is REALLY good for a 4-stroke gasoline engine. 
As Andrew points out correctly, there is NO way to convert it to MPG, and whoever says there is is lying or pulling numbers out of his behind. Example: small 35% engine in super aerodynamic 3-wheeler: 2L/100km, same engine in a Hummer will probably do 20L/100km -> if you believe it can be converted, you don’t get it 