Quote Originally Posted by Adondo View Post
On edit: I've reset the speedo with the Hyperchips programmer, so it matches the GPS. That means it REALLY DOES get as crappy of MPG as the computer shows.
Yeah I've noticed that I always have to put in a smaller tire size to get the speedometer accurate since the tire flexes with the vehicle weight.... so for my 245/75/17 that are supposed to be 31.5" tall I had to put in 30.5 to get my computer to read accurately. That works out being right too if you follow most programmers instructions; they say to measure from the center of the hub to the ground to get the true "effective" radius of the tire and double that for the programmed diameter.

A lot of people just put in the tire diameter based on size... always makes me wonder if their reported mpgs are a bit inflated because of that... (though its only about .5 difference in most cases. Still food for thought)