Okay. Original sd 2011 with 6.2 L gasser came stock with tiny 17 inch wheels and tires. I noticed that when I installed 35 inch tires (18 inch rims) that the speedometer seemed to still read correct. I have an EDGE that I use to display both MPH and corrected MPH for the bigger tires. Plus, my GPS corroborates the speedo and EDGE-corrected speed. Am I wacko or does this truck figure out what tire size is on it somehow? It is reading the correct speed according to my GPS on the speedo. Please set me straight.

 

I also have a 2011 6.2L. I changed from the stock 275-70-18's to Nitto 295-70-18's. My GPS and the speedometer are reading the same now, stock the GPS would read slower than the speedometer. I did notice the MPG decreased with the new tires (more than I thought it should have) I took the truck out on the highway, reset trip B and did 4 mile markers the speedometer read about 3.8+ a little. all the math works out to be 3.5 to 4% off. I have an appointment Tuesday at the dealership to reset the computer for the new dia. tires. I will let you know what I find out Tuesday.

 

Well mine reads 3 mph slow. So maybe with the bigger tires it will read correct now.

 

I went to 35's as well and i went to the dealer to calibrate it, 295 is the biggest they had and my speedo reads slow according to my gps, and it multiplys past50, at 50 im doing 51- gps at 65 im doing like 70 i Belive, ill look to make sure, i wemt from 18mpg to 15-16 hand calculated with the gps mileage i according to it im putting less miles on it than i actually am

 

I just got back from the dealership. The tech. hooked his computer to my truck to check for the available tire sizes and it only showed sizes that must have been available on our trucks. The truck came with 275-70-18's and that was the biggest 18" it had listed but it did list a 275-65-20. So I asked about the 20" tire and he said it was not any bigger and I left with nothing changed. I came home and checked the 275-65-20 Michelin at Tire Rack and it shows a diameter of 34.1" I might go back and see if he can program that size into the computer. He also told me that the main concern with larger tires is the way the transmission will figure out its shifting, but at 3.5 to 4% I will have no problem, it is when you get around 15% difference that things can go wrong. There was another place he looked and it listed the axle ratio's 3.31, 3.55, 3.73, 4.10, 4.30, and 4.88. I don't know if it would work but if I did the math right a 37" tire with 4.30 gears would need a 4.80 axle ratio set in the computer to bring the speedo back to correct so a 4.88 setting might work for the guys with the big tires. But if the setting for 275-65-20" will work on my truck that is the closest I can get 34.1" and the largest dia. available.