O.K. A 285 has a diameter of roughly 33". When you mount it on the truck, it squishes down to 31.5+/- Do you set your programmer for 33" or the squished 31.5'?


Thanxhttp://www.dieselplace.com/forum/smileys/Big Smile.gif

I would think that the circumference of the tire (the distance around the tread) has not changed. My vote would be to use 33".


If you want to get it exact, set it for something (say 33") then take it out on the road, set the cruise and monitor speed with GPS. Then tweak.

Thanx for the response. Looks like we're from the same area. I live near Esperanza and Paseo Del Prado

Definitely 33". It's the size of the tire unmounted that you use.

The proper way to calibrate the speedometer is to take the rollout distance of a mounted tire/wheel combination. To do this mark your tire at the point it hits the ground and the ground. Drive straight forward until the tire makes two revolutions and the mark on the tire is now hitting the ground again. Mark the ground and then measure between the two marks on the ground. Take this measurement and use the following formula to get the dia. of your tire/wheel combination.


(X/2)/3.14159265359


An example would be as follows


(208"/2)/3.14159265359 = 33.10423"





If your programming tool is working properly this will get your speedo perfect!

YoKev,


I am out in the east end of YL in the canyon. La Palma and Via Lomas de Yorba.

I thought the stock computer in the Duramax can only be calibrated to 265's. How are you guys calibrating yours for 285's?

Mainemax-If you have a programmer like a quad stealth, predator, hypertech, or bullydog they can all set tire size. They can't alter shift points for the allison but they can set the speedometer/odometer and abs for larger tires. And of course add more power.http://www.dieselplace.com/forum/smileys/Big Smile.gif

Mainemax, when they say that they are refering to the dealership. They can only adjust up to 265's if you can find one who will reprogram it.

So you are saying a dealer will not adjust for 285's for liability reasons or such, or that he cannot physically do it because of not haveing the right tooling??


Reason I ask is, I know a guy with a OBD 2 Scan tool. Would he be able to adjust for 285's?

Actually I don't think they have the programming to do it in them. Ask your guy and see. May get lucky!