I have a k1500 suburban. We bought it cheap for a winter vehicle despit it having some major flaws. The speed is off, the difference starts off low like when doing 18 we are actually doing 15, when we are doing 65 we are doing 55. The tact is reading what it should if we were actually doing that speed on the speedo, but were not. I have a code reader so that's how we know what's going on. Does anyone have any clue what this can be from? Every one we talked to says tires but it has 245/75/16.