I went from 245's to 265/70r18's knowing my speedo and odo would be out by around 6.5% according to the math. The other day I had my Garmin GPS in the truck and at 100kph, the GPS showed I was actually doing around 106-107kph. 'Very good', I figured. That was the expected difference. Yesterday I did a highway run and decided to compare the odo. After 10km on the odo, I did 11 on the GPS. After watching it for 50+kms, I confirmed the difference was 10%. But the speedo difference was still ~6.5%.
Could the odo and speedo in the truck be out of sync? If indeed I am getting 10% extra distance versus 6.5%, my gas mileage is not as bad as I thought it was.