I got my truck about a month ago, 2003 2500HD crew cab short bed all stock on the suspension/body everything. It was flash programmed before for bigger tires and the speedometer was changed as well. When I am going 50 on the speedometer I am actually going 45. AND the odometer is off as well, for every mile I go I am actually going 1.1 miles, as you can see I want it to be right on, I am due for new tires and I want to get the closest I can to what ever tire size this was programed for. It has always been stock for as much as I know. I am not looking for width as much as I think I am looking for height of the tire maybe..... I just want my readings to be more accurate.... my odometer is already off so much as it is........ HELP.......

Not sure this will answer your question, but I have the same truck as yours, only it is a 2005. I had a problem with a knot in the sidewall of one of my original Bridgestone 245-75-16 tires. Rather than purchase one new Bridgestone, I upgraded my tires to BFG AT/KO 265-75-16s, and expected that my speedometer would be off due to the tire diameter changing from stock. I put my GPS in the truck and noticed at 45 MPH on the GPS, the speedometer in the instrument cluster was at 45 MPH, which puzzled me. The truck speedometer also seemed to be right again at about 75 MPH. The speedometer was not on any other place on the dial. That caused me to wonder if the instrument cluster speedometer had some kind of bias at that point of the dial, (x degrees off of straight up). I never checked the speedometer against GPS with the stock tires, so I can't say if it was right on for all positions or if it was off at 45 and 75 MPH, but found it curious. I wondered if your speedometer is OK at other speeds, but just off at 45 MPH, and perhaps at 75 MPH? If so, the speedometer may not have been adjusted for larger tires, it may just be inaccurate at that position on the dial.

Thank you I will have to check that out!!!! Thanks again very much!!!

LT265s corrected the speedo on all my LT245 equipped trucks, with ZERO programming changes.

Awesome ok thanks that is what I will have to get then.... Thanks!!!! :)