I have heard you can buy speedometer correctors for use when putting non stock size tires on your truck. Back in the cable driven days, which a 91 might still be.. idk, there was just a plastic? gear on the end of the cable that you could buy with several different tooth counts to help get your speedo accurate again. Better to get one off a parts truck instead of at the dealership... they wanted $42 for the last one i inquired about!!