If you change from (factory installed) 265/70 R 17 tires to 285/70 R 17 on 2007 2500 SLT 4X4 how much would your speedometer be off by? Can this be changed in the PCM to reflect the difference in ratio's or is not enough difference to worry about? The power wagon comes stock with the 285's.

 

Your speedo will be off by roughly 3.4%. 68=70.3 mph

 

It can indeed be changed in the PCM to reflect a change to LT285/70R17's.

Greg

 

Greg, is it very expensive or worth it or a easy update?? Just looking for some advise.

 

Depends on the dealer you deal with.

On my previous 2003, I had the pinion factor changed at my dealership for $30.00.......a little less than .5 hr labor rate per hour.

I've read on here that some dealers will charge $75 or more to do the work.....which takes all of 10 minutes or less to perform.

Greg

 

If you add the taller tires the speedo will be off in your favor why not leave it. No one is going to stop you for being 3% over. Unless your like me and its going to kill you knowing its not exact.

 

When my truck was in for another problem I had them change my speedometer programing. I put 295/70/17's on it, they could only get to 285/70/17 in the software. Stock size was 265/70/17.
Now my speedo is off by 10%! Before the change it was only off by 4% or so. I'm going to have them change it back to the stock setting. I'm thinking that there is something not right with the software programming for the 285/70/17 setting.

WHY DID THEY WRITE THE PROGRAM THIS WAY?? THE OLDER TRUCKS CAN BE DIALED RIGHT ON!!

 

I've found this tire calculator to be pretty handy.

http://www.discounttire.com/dtcs/infoTireMath.dos

 

How does it make the MPH calculation? ABS tone rings (I know 99-2004 Jeep Grand Cherokees do this) or by a gear in the transfer case?

I used a device to dial it in on my old Jeep and it worked pretty well. Superlift TruSpeed

http://www.superlift.com/accessories/truspeed.asp