This might be a dumb question and maybe I havent noticed before but after I drive for awhile my overhead console will say like 13.7mpg. I will park it for the night and start it in the morning and it still will say 13.7mpg I go inside and let it warm up for awhile and when I come back out it says 12.1mpg. Is it taking the idle time mpg and averaging it out??? Is it supposed to??? I might have never noticed it because I never let it warm up in the summer. Any thoughts???

 

It compares mileage driven to fuel consumed. When it's just idling, the mileage driven is 0, so the average mpg, in this case 13.7, can do nothing but come down.

Like the old saying, idling gets 0 mpg.

 

Yea the computer doesnt care if your driving or not, if the engine is running it's using fuel. It doesnt difrientiate (how ever that word is spelled!) between idle time and driven time.