SmokeyWren has the exact formula for the dealer tech to use. Maybe he'll chime in.
Toyo LT305/55R20 tires are 625 tire revs/mile. Click here.
So most Ford dealers probably have someone that can calibrate the speedo to 625 revs/mile.
The following procedures are several years old. Ford techs have newer, better diagnostic tools to use now than when this first came up for the '99 models. The WDS has now been replaced by a newer diag system. But maybe print it out and give it to your tech anyway. If he's been around more than a few years, he'll probably be able to adapt the following procedures to whatever is the newest diag system.
If you dealer's tech hasn't calibrated a speedometer on a SuperDuty before, he might spend an hour researching on how to do it before he actually does any work. After he has done a couple, then it's a 5-minute job – but most dealers will charge you one hour of labor to do the job.
If your service writer sounds like perhaps that dealership is not experienced in calibrating SuperDuty speedos, then copy and paste the following into a Wordpad or Word document, print it out, and give it to the service writer ot tech.
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. ]
• [ADDED #2: Ignore the above "ADDED" if you use the newer NGS Plus. With the NGS Plus you can calibrate the speedo on any 7.3L or 6.0L PSD.] [*] [Added #3: Some techs now use the Ford Worldwide Diagnostic System (WDS) to calibrate the speedo. I don't know anything about how they do it.][*] Oops! Time flies and the WDS is now obsolete. You're on your own with whatever diag system is now in use.
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.
• [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
For 305/55R20 tires emter 00625
Do not try to enter a number less than 00601 with the old NGS. With the newer NGS Plus or WDS, I don't know. ]
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.
Some Ford dealers will refuse to calibrate the speedo. They say it can't be done - or that they're not allowed to do it. So in that case just refer them to the 2003 Ford gobbledegook version of the procedures at: