put 34's on my truck and took it in to dealer to change the revs per mile to 609. They tell me they cant do it but will somehow try. This is real stupid, why could this not be done? anyone else have this problem?

 

My son is a chrysler mechanic and when I went to larger tires he came home with this thing that plugged in under the dash and as we were driving down the road he fixed the speedometer. And in the winter when I put stock tires back on he already knows the code and it's done. NO problem. Get a new mechanic.

 

Hard to imagine, but if the mechanic rarely sees this request, you can let them know that to get to the tire size function you must choose "pinion factor" from the scan tool menu.

 

My dealer had zero problem when I changed up to 315's. As he already had the computer connected doing other stuff, he did not charge for this change.

 

If I recall correctly, Chrysler did something to the computators on the 06 (?) and up trucks that wouldn't allow the techs to adjust for tire size changes. I think the only way to do this on an 06 or higher truck is to a) get a Smarty or Smarty Jr. (someone correct me if I'm wrong), or b) do a search on this site for a thread about being able to change the revs per mile or pinion factor using a Snap-On tools code reader....

I was wondering if they made this option available again on the 2010 trucks or not - guess they didn't, or the dealers don't know about it....

 

Just get them to program it to 285 which they should be able to do, it will be pretty close. If you had 35s and 4.10s they could just adjust the computer to 3.73s.

 

ok guys, the later model trucks dont have the option to just change the rev per mile. MY old 04.5 did but the newer ones dont. The newer ones can change tire size based on factory tire sizes available and change gear ratio. From what the dealer told me and another tech from somehwere else, approx 635 revs per mile is the lowest thay can configure it in the computer. this is approx a 33" tire. This helped my speedo as they told me my speedo was approx 685 revs with the tires size gear ratio combo. IF anyone else can 100% confirm that it can be adjusted further then please let me know

 

Also revs per tire are usually off as per the manufactuer's spec and the speedos read a little high so I'd grab someone's gps, tomtom or something and verify how far off you are.

 

Have you seen this?

http://www.miata.net/garage/tirecalc.html

 

That's interesting. It says with me going to the 285/70/17 if I'm going 60 mph, I'm actually going 62.1 mph. I have noticed a little difference in the shifting. I wonder if I still would need the speedo recalibrated?