Your Tire Size (TS) is way too large! I'm surprised it's only 4 mph off. A larger circumference makes the PCM think you are moving faster than you really are.
Bill has this neat "Gear Ratio Calculator" in Pegasus that I use for TS. All you have to do is put "315/70-17" (notice the dash in front of 17) into the Old Tire Size box. You will immediately see 2672 mm in the Old Circumference box. Notice the "Squish" bax has a "5" in it. The circumference is being adjusted for a 5% "rubber squish" (what Ford uses). I get better results with 7%, so if you change the "5" to "7", you'll see 2644 mm.
Try either of these values, and adjust as necessary.
- Jack
Last edited by Jackpine; Sat, August 1st, 2009 at 02:29 PM.
Reason: Fixed stupid typo
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Fri, July 31st, 2009, 04:57 PM
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