2003 QC 4x4 5sp 4.10 rear.

Truck had 305-65-17 tires on it when I got it and was 6 mph off at 65 mph.
I went back with stock size 265-70-17 and the speedo is still off the same 6 mph at 65 mph.

What would cause that?

I would have thought the speedo would be right going back to the stock tire size. Heck, I should have seen SOME differnce, but nope, same thing.

Any clues?

 

well there is only about an inch difference on diameter for those tire sizes. That is new tread on both. Worn 305-65-17s could easily be close to an inch smaller diameter when on the truck. Tires in general will have an actual smaller mounted/rolling diameter than specs. And then the speedometer is not exact either.

 

fade,

You can get the pinion factor adjusted at the dealer. If they are reputable they will probably do it for free. Takes about 1 minute and the right electronic tool.
Do not pay an hour labor or listen to them tell you it can't be done.

Or, get a Smarty and do it yourself.

 

Local factory service outlet wants $60 to do it.

 

i just took mine to the dealer to have it done and they charged me 49.95 and i bumped my speed against my gps and it is still off i wonder if they even did anything but move my truck around and keep me there for two hours

 

Once again the advertised specs on tires, rpm, diameter, etc. are based on design factors. Put a tire on a wheel, air it up to spec, put it on a heavy vehicle and the actual rolling circumference and diameter will be different. I have had many different tire sizes on this truck. I had the dealer set ABS pinion factor a couple of times based on manufacture spec. They were close, but never exact. I have used Smarty, BDTD and DRBIII to set it myself since then. The only way to get close is with a good GPS and something to reprogram with. My current BFG 285/70R/17s which the BFG specs say are 32.7" diameter I have set at 32.0" with the Smarty. This is pretty close to exact with GPS. They have 40k plus on them now. New 32.3" was close to exact. I am pretty **** about this so I check speed/distance with a GPS at least every 1000 miles or so. When it gets to far off I reset the tire size again.

 

I'll eventually get a Smarty, but I have other things to do to the truck before then. I want to get a water/meth system prior to the Smarty.

All the info from ya'll makes sense.

 

More accurate way of setting is revs per mile. 305-65-17 is 618 revs per mile. 265-70-17 is 638 revs per mile. Depending on year of truck revs per mile can be used instead of tire size. Some late model trucks can only have select tire size calibration used, if tire is not listed closest match will need to be used.
Link for setting REVS PER MILE
http://www.gordon-glasgow.org/tirecalc.html


Check for proper pinion factor ( tire size ) programmed into CAB module. The correct pinion factor (tire size) must be set into the antilock brake control module (CAB) with DRB diagnostic tool for accurate speedometer operation. The CAB module sends tire revolution per mile information (mph) to powertrain control module (PCM) for proper operation of the speedometer system / transmission governor system / other engine functions. See DRB diagnostic tool braking system menu for details. Select the tire size that is currently installed on the vehicle. Only factory installed tire sizes are supported. No pinion factor is available for tires larger or smaller than those offerd by the factory. If two or more tire sizes share the same pinion factor all tire sizes for that particular pinion factor will be shown on the DRB tool display after the pinion factor selection is made. This is normal condition due to the similar diameter between certain tires used on DaimlerChrysler vehicles. NOTE: When selecting pinion factor on (DR) model the DRB menu may request number of teeth on the RWAL sensor ring inside the rear axle. The correct entry would be (108) teeth. P245-70R16 TIRES: This tire size may not be listed in the DRB tool display. When setting pinion factor for this size tire select other on the DRB tool menu then enter (711) revs/mile to aquire the proper pinion factor.