I have a 91 with 4:88's and 315's, I have checked my speedometer and odometer against many radar signs and a few different gps units. My actual speed is always consistently 5mph below whatever my speedo reads, if it says I am going 25mph the radar and gps read 20mph and it's the same at 75mph, the radar and GPS say 70 mph. My odometer is off by 1 mile for every 33 miles I travel. If I pass 32 mile markers on the interstate or log 32 miles on the gps than the odometer reads 33 miles. My rig has consistently been this way for the last 6 years. Based on this my average mpg's in the city are 11.8 and 13.1 for highway. I don't know if the previous owner did anything to correct the speedo and I am wondering if anyone can tell based on these numbers? Or maybe someone could explain how to check to see if the PO attempted to correct for the bigger tires and regear? It had 35 x 10.5 x 15 when I got it. Thanks