I understand that anytime a tire or tire and rim/wheel size changed it can throw the speedometer off. My truck came from the dealer with 235/75R16's on it back in 2000. Since then the rims are original but when those original tires wore out, I replaced them with 265/70R16's and that is the size I have used ever since. So I plugged in 265/70R16 into the intune and I am noticing the "miles per fill up" go up, I am sure that part of this is because of the custom tune, but how is the change I made as far as the tire size goes going to affect how many miles I am getting per fill up now? I am calculating the miles per fill up by a combination of my trip odometer and records I have been keeping about how many gallons each fill up was, what the odometer read at each fill up, etc. Thanks, Nak.