rj'sjeep

I was curious so I measure my tires. I have a 2008 rubicon 4 door. this is what I found. On the original tire which is listed as a 255-75-17 it calcs out at a 32.1 according to a web conversion site but it actually measures 31.0 I put on new Falken wilderness at which calc on the web site at 32.7 but actually measure on the ground at 31.5. so what would happen if you had a super chips and programed it for a tire that is say 35" when it actually measures 33"? would you get more apparent power or what? Just Curious maybe one of you has a superchip programmer could run a little experiment and tell us what happens.

jcs07jk

I believe programming tire size into the superchips only has a effect on the speedo adjustment. Now if you had a true 35" tire compared to say the bfg km2's that measure out on the smaller size, I would say that there would be a very slight performance gain from the smaller tire only due to weight and the slightly smaller diameter. I don't think you'd notice it If you install the superchips performance tune that's where you get better performace and setting tire size helps with auto shift points. I originally installed my superchips with 35 tire setting and realized with my gps that my speed was way off. Dropped down to 33.5 to get speed close but felt just the same driving

GCM 2

x2, knowing the measurement of the tire is only necessary for speedo correction. After my gears and tires were changed, I found that by plugging in the exact new gear and tire size with my Flashpaq, my speedo was still off by about 5-7 mph. This took a few tries, but basically I used my Garmin GPS to monitor my speed compared to what the jeep was reading, then made adjustments on the Flashpaq after each run until both speedo and GPS were reading same numbers. I ended up plugging in 38.75" tires (even though I have 40's that actually measure 40"s) with 5.38:1 gears. I have confirmed my speedo several times around town with the radar speed limit signs that flash your actual speed below the sign, and speedo in the JK is spot on.