I've personally never found the car's computed mpg to be off by enough to bother with doing it the old fashioned way for years. My GTP was accurate and was the SS. The car's computer can take into account extended idling and the like...it's 10000 times more capable than a calculator after all. LOL!
Call me lazy but I'm not going to worry about it being off by a .5 - 1 mpg if it is all.
