There is always some measurement error. Most people complain it's the other way around. In mine, it's the other way. Over the last 13K miles, the trip computer says I've used 286.9 gallons, but the pumps say 277.4 That's 1.5 mpg under actual.

 

My Chevy is the same. I think they calibrate them that way on purpose.

 

Agree. They can't make them 100% accurate, and +/-2 mpg is an acceptable range. And if it is -2, it becomes an "issue" where it seems they are trying to deceive you, but +2 is just measurement error. So they intentionally overestimate gallons, and underestimate miles (keeps people from speeding too much, as well).

 

Mine reports 1mpg under actual used.

 

I've heard of people using phone apps to try to compare, but I don't know if it's any better...I'd like to think the car knows best. I guess if all else fails, a pen and paper in the glovebox like my grandfather used to do will work just fine.