Speedometer Calibration
 

I checked my Speedometer/Odometer calibration last weekend and it was off somewhat. I haven't run the numbers yet, but before I change anything I'd like to make sure I understand what I'm doing.

I have a Gryphon with canned tunes on a 1999 F-150 4.6L. Haven't ordered my custom tunes yet. I loaded and am running the towing tune, having tried the transmission only tune for a day or two. I HAVE NOT entered the custom parameters menu.

As I understand it, the Gryphon has the ability to change the speedometer calibration. I think this is done by changing the tire size. I think that changing the tire size will change all of: the Gryphon speedometer, the dashboard speedometer and the main and trip odometers.

1. Assuming all of the above is correct (?), and I am running the canned towing tune, what tire size is being used? The original tire size stored in the VID(?) area of the ECM, or a default number that is loaded with the canned tune? I'm asking because my odometer is off now but I never checked it before installing the Gryphon.

2. How can I determine what number the Gryphon is using currently in the canned towing tune?


3. Assuming I can get that number, I presume I can adjust it by the same percentage that the odometer is off and that should correct the odometer and speedometer, right?

Thanks for helping the newbie.....

Al

I went through this exercise about a month ago, you may want to read through this thread as all my ?? were answered there: http://dygytalworld.ehost-services13...ead.php?t=1437

If you are using the standard tunes with no custom modification, the PCM is using the VID for information. The Gryphon (and Evo) currently do not look at the VID information, instead using the values in the PCM "placeholders" as a base for the custom options. What I might be able to do is actually add a diagnostics function that displays the current VID information which you can then use in the custom options.

I'll have to add that to the "do" list.

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