After adding new 285's on my 1500 I used the Superchips to adjust the speedo, I set it at 32.75. Using the GPS today I see my speedo is reading 2-4mph under the actual speed. Should I adjust the tire size to 33 or down to 32.50??
Superchips specifically measured the tires from top to bottom while the vehicle is on the ground. Don't measure side to side or use one of the online calculators. Just get a tape measure out and measure from top to bottom. I know it's not truely realistic, but that is the way the programming in the tuner is set for.
I noticed that 65mph didnt feel the same when I mounted the nitto 308/60-r17's . The mfg said the tire was 31.4 and the tire clacs all said 31.225. I had it set in the middle and it didnt feel right so i measured them and came up with 30.something. I changed the speedo to match and now all seems right