Speedometer Calibration Spreadsheet

[lwillson]

After reading the post

1994 speedo calibration

and similar ones, I wanted to make a spreadsheet to do all the math for me.

Hope it helps anyone who wants to change their calibration number

[pjwoolw]

I thank your sir! That will make things allot easier for many people.

[CT Performance, Inc.]

thanks!!! great idea.

[Wisner955]

With all due respect for all the folks here I would like to add my experience.

I have 1994, F150, 300 Six, 5 speed stick, 2 wheel drive.

None of the information in these posts would apply to my truck. I ran the numbers and also looked at the spreadsheet listed. Going by the numbers here I came up with 1089 calibration number. That gave me 70 MPH at an indicated 64.5 on the speedometer - way wrong. I kind of figured it was not calculated right when I saw that the number was nearly off the chart with 28" tall tires.

I found the correct information here: (Reseting Recalibrating your PSOM for the Correct Speed)

As you will see it has completely different numbers from what is presented here and works correctly on my truck.

Hope this helps.

Paul

[lwillson]

You are correct, the numbers will be different.

When I made the spreadsheet, I used the calculation listed in the other post. I had no idea if it was correct or not, I just used that formula to derive the spreadsheet.

Having said that, the calculations also are based on the larger 3/4 ton rear end, which has a larger differential, therefore more teeth on the tone ring. The calculations will be different to compensate for the number of pulses that the PSOM has to calculate to determine a complete revolution. That is why the numbers will be off compared to a F-150 and a Bronco.

[weareweird69]

So uh, where is this Module that needs replaced? After changing the speedo 6 times it needs replaced...

[lwillson]

The PSOM module is the speedometer itself. If you are patient, you can just change the speedo from the instrument cluster, but it is easier to just swap out the whole instrument cluster

[weareweird69]

Oh, Ive done that before, but I didnt think you could change the mileage back to correct then?



Is there a way to change the mileage displayed? Or can the dealership do that? Ive got another speedometer here just in case.

[Wisner955]

Iwillson,

Thank you for the reply. Good to know someone else is reading these things.

I understand that you used someones "formula" and created a spreadsheet. My beef is with the original formula and numbers - I've tried to run them every which way I can think of and see no explanation for "20186" and the ".6666" or "66.66". I'd like to have Woody 1 walk me through the basis of 20186 and .6666.

Speedometers are the same for the trucks and it can only be 1) Revs/mile (tire size), 2) Number of tone ring teeth (108 or 120), and 3) the constant (8000 or ?).

I just don't want folks later on to get frustrated and waste time like I did using those "numbers"

Best Regards,
Paul

[weareweird69]

Here is some more information for you guys

7.5" Axles have 108 teeth on the ring
8.8" Axles have 120 teeth
10.25" Axles have 120 teeth
10.50" Axles have 120 teeth
11" Axles have 120 teeth

I also found this...

TOD = Tire Outside Diameter

If TOD is greater than 25.2", use Formula Revs/miles = 1528 - (28xTOD) to calculate SAE average revs/mile

If TOD is less than 25.2", revs/mile = 10,850/TOD.