Speedo Correction for Larger Tires
I got the following info from Heber at SMB to reprogram the van computer to make the speedometer read accurately for 315/70-17 tires.
The numbers we use to recalibrate the PCM are as follows using the PDS diagnostic tool. Select module programming, then programmable parameters tire size/axle ratio.
The column that will be selected for tire size will be marked OTHER. This will open next window where the tire size will be input.
Input tire size 315/70-17 (the rpm value that will automatically be calculated by the computer will be changed). Input new RPM value 699. Select axle ratio 3.55.
If the technician needs more info he can give us a call and we would be happy to help.
This will only work for this size of tire, but I'm sure SMB has more code combos for just about any tire you could put on an SMB. I don't know if you these changes can be made by a code reader type device (like a Scangauge). I had the local Ford dealer do it for $93 (min charge for 1 hr labor, but I'm sure it took the tech 10 minutes).
Before going this route I considered buying an electronic device (either a tuner or the specialty speedometer module), but none of the manufacturers could definitively say if it would work on a van. The van has a different PCM programming than the F-250/350/Excursion and many of the aftermarket products just were not designed for the vans.
2007 Ford E350 PSD Quigley 4x4 and El Kapitan conversions, 315/70-17 BFGs