I recently upsized the tires on my '04 Suburban 2500. I went from the stock 245's to 265's. This increased the circumference by about 1.5" to 31". This change put my speedometer/odometer off by about 5%. At 70mph on the GPS, my speedo is showing 67mph. I have also done a couple of measured miles on the highway, and the odometer is also shy by about 5%. I have been to the GMC dealer twice and asked the to make the proper adjustment. They said that it was done, but the speedo is still off. Are there any suggestions out there as I am about ready to go to 265's on my '05 Yuk XL 2500. Thanx.