The Official Superchips Forum

Question? I have a 05 Hemi ram with the 3815 AND LOVE IT! However I switched over from the stock 17inch wheels witht the stock tires to a set of 18*9.5 wheels with 285/60/18 tires.. My question is. My superchips tire setting now is on 30.75 for the stock tires. QUESTION should i measure the new tires while they are on the truck under the weight of the truck or should i take a wheel and tire off and measure it with no wieght on it. I measured one under the wieght of the truck and it was 30.25. thanks.. SUPERCHIPS KICKS *h! love the 2nd gear chirp!

I would say that it would be best to use the manufacturer's website for whatever tire you have instead of fiddling with trying to measure it. Most tire manufacturers do a real good job in describing all characteristics of the tires they make. Try that.

In case that's not available, and you have to resort to measuring, then I would think it would be best to measure the new tires taken off the vehicle and with all of the air let ouf of them. I've tried to do this before and it's not as easy as you think. Take something like lipstick and radially mark the tire and then put a piece of paper under it and roll it until the lipstiick makes its mark. Then, roll the tire around and get ready to place another piece of paper for it to mark again. Just measure from the center of the two marks and you've got the tire's circuference (sic - editor did not like correct spelling). Take that measurement, in inches, and divide by 3.14 to get the diameter. Round to the nearest 1/4" and you've got it.

Plug that number into your FlashPaq and you should be good to go. If you have access to a GPS receiver, then you can absolutely dial the speedo to within 0.1 MPH if you' like to fiddle. I can vouch for the factory speedo with 20" wheels being pretty much dead on at 55 to 60MPH.