When you change tire sizes such that it affects the speedometer, is it just a simple computer program change done at the dealer to put them back in sync, or something more involved?

 

Some users have reported that the dealer was able to change the tire size programmed in to the truck and thereby recalibrate the speedo.

A lot of folks have commented that dealers have been unwilling to do it for the most part.

 

Had the dealer change my '02. It took them about an hour to do. I had watched a chevy dealer change my wifes tahoe and all he had to do was scroll through his programmer and pick the right tire size (about 15 min) so when I took mine in to ford thinking it would be a quick in and out I was informed that with mine they had to take the exact tire dimensions, do some calculations and figure out which thing to input on the scanner to change it correctly. The mechanic was not thrilled with doing it because he said it was a lot of work. Maybe he was math challenged? Don't give up, it can be done!

 

All you have to do with the Fords is change the revolutions per mile number in the computer. (dealer item) You can go as low as 601 which is about a 35" tire. Should take an experienced tech 15-20 minutes. Read this: Here are the detailed procedures: *copied from a post at TDS, thanks Smokey*

1) Ensure all harness connections are plugged in.

2) Using Rotunda New Generation Star (NGS) scanner and Version 3.2 or higher Service Card, plug into the DCL connector.


[ADDED: Do not use a service card version later than early 2001 model year. This will not work if you try to use a service card dated after the 2002 models came out. But it will work on 2004 or earlier trucks with PSD engines if you use a Service Card for 2001 or earlier SuperDuties.]

3) Turn Key on w/Engine Off.

4) Select Service Bay Functions

5) Select ABS Module or 4WABS Module depending on application.

6) Select Tire Size/Axle Ratio Configure.

7) Enter tire manufacturer's specification of revs/mile (tire revolutions per mile) into the NGS as the REV.PER MILE figure, preceeded by two zeros.
[ADDED: To have an even more accurate odometer, enter the following rev/mile:

For 285/75R16 tires enter 00636
For 295/75R16 tires enter 00626
For 315/75R16 tires enter 00601

Do not try to enter a number less than 00601.]

8) Select Tone Ring size and verify that 120 is programmed in.

9) Select <Option> and press TRIGGER.

10) Select <Vehicle> and press TRIGGER.

11) Press <Done> and the Module will be reprogrammed.

12) Check Speedo accuracy.

To determine your odometer’s accuracy (if you don't have that fancy newfangled GPS stuff) just find an interstate highway that has two mile markers 10 miles apart without being near an entry, exit, bridge, or overpass. When your front bumper passes one of these markers, push the odometer reset button to zero. Ten miles later, your odo should read exactly 10.0, which is 100 percent accuracy. Every tenth of a mile off from 10.0 is one percent error.


The Ford gobbledegook version of the procedures is at:

http://www.fleet.ford.com/truckbbas/...bb_pdf/206.pdf

 

Thanks fellas, I appreciate it.

 

good post, so this would eliminate the need for that little aftermarket box a lot of people buy to recalibrate? Does it fix your transmission shift points to work better for the tires too? Doesn't the aftermarket box have that feature?

 

Great post! Just what I needed. Thanks v10!

 

Your dealer should charge you $30-40, I believe it is what an hour of labor rate would cost.

 

1. No.Older units need the black box. ECM can not be recalibrated.
2. Yes. Recalibrating the tire size will help shift point.
3. Don't know..

 

I found a formula to calculate Rev per mile on a link:

Measure the tire height from ground to the top of the tire in inches. Ensure an accurate reading to the nearest 1/8 inch. Divide 20,168 by the tire height in inches to get the revs per mile.

Example: Measured tire height - 33 inches
Revolutions/Mile = 20168/33 = 611 Rev/Mile

So my question is to v10man, why can't you enter below 601? According to the Ford manual you can use preselected tire sizes or type in a custom revs/mile? Please provide more details. My 315 tires are 34.5 inches tall, so based on the calculation my revs/mile = 585. Why can 00585 not be typed in? I am taking my truck in tomorrow to have this done.

 

Unless I am wrong.. can't the dealer only adjust it for tires up to 35" or so?

 

FWIW, the SL Truspeed is awesome. easy to install and calibrated to what i would say is 99% correct.

 

601 is the lowest the computer can input, that was the best the dealer could do for my truck when they did it. They said that Ford deliberately made this the lowest setting because they didn't want to look like they sanctioned larger tires, because of lawsuits I would guess.

 

My dealer wouldn't do it. They cited the fact that all this information is transferred to Ford and with the warranty wouldn't make an adjustment for a vehicle that was beyond original specs.
This is the first time I have had an issue with my service dept not working with me.

 

My dealer himmed and hawed when I asked them to do that for my '02. Found out that they just didn't want to work that hard. They had to figure out what to input from the tire dementions and were not quite sure how. Took about 1 hour for them to complete.