88' bronco w/ AOD tranny and 31x10.5 tires, Is there a specific speedo gear that I can get for a specific tire size that will recalibrate my speedo to the correct speed? speedo reads 60 and actual speed is 65 or 66, this makes it hard to check my MPG's, I'm getting ready to do some upgrades and would love to be able to check befor and after MPG....