Speedometer Calibration

[Jackpine]

Al, what you posted in your question was correct. If you did not open (notice the emphasis on that word) the Custom Options menu, the PCM is using GR and TS information that was placed by Ford into the VID section of the PCM.

The Mad Doctor once said that Ford uses a 5% squish factor here. I've found that subtracting about 3% from the unloaded circumference of the tire is more accurate (multiply by 0.97) and suspect they use 5% due to people's tendency to drive on under inflated tires.

If you change the TS and set the proper GR, you will effect the dash speedo, and the odometer rates of change. You cannot go back and change any values that were previously recorded though, so if your total truck mileage is 115,000, it's still going to be 115,000 after you adjust things. If you can get the Gryphon's speedometer to read dead accurate, you will find the dash speedo reads about 2 mph fast in the 70 mph range (about a 3% error there too), but the odometer will be accurate.

Adjusting TS by the same percentage that your speedometer (or odometer) is off is a perfectly good way to do it. But, there's really no way for you to know what TS the PCM is actually using, as Bill said. You MAY well be running on non-OEM sized tires, if your truck is a 99, and that may be the source of the error.

I'd use the Gryphon Manual formula for tire sise and multiply that by 0.97 to get a starting figure.

- Jack