No sense renting/buying a spendy GPS just to see what your speedo is off. Do what PEH said and use the mile markers over a least 5-10+ mile stretch or find out what size your stock tires are in dia. Call a tire shop or search the internet for dia of the originals and of the new tires. Basically the % increase in tire dia is going to make your speedo/odo read slow by the same %.