Calculating Correct MPG

[dandul]

Hey Every1

I noticed that my speed-o is off by 5mph ( checked it with a gps ? ) question is I was just simply dividing the miles driven on the analog counter by gallons of fuel. Should I be doing it another way?

[2003kingranch]

mine is off 5mph also so actually im getting better mpg then what im hand calculating i was going to change it in the tuner but i dont know how?

[Dietoremain]

ya mine seems to always be about 2-3mph off(according to the speed signs on the street that say 'slow down' lol.