dosn't the axle ratio need to be figred in also?? I would figure that that would make a differance. I did look at mine and i have the #6 at the end so I am assuming that it has never meen changed.
Thanks for the help

 

The Gear ratio doesn't matter with the electronic speedo's because the sensor is mounted in the rear axel not the trans like the older cable driven speedometers.

 

Oh ok thank you so does anyone know what the revs for a set of 35-12.50x16.5's might be?
I do appreciate all you help

 

contact the place you bought them at they should have it in their books.... i was told the formula by the dealer here but i forgot it already.. opps

 

multiply the tire diameter times 28. subtract the result from 1528.
multiply times 108 (rabs) and divide by 8000 equals the constant.

31" x 28 = 868 1528-868= 660 revs/mile x 108 =71280

71280/8000 =8.91 constant

this is for tires larger than 28". the constant in the ford manual is 9.18- revs/mile of 680. the tire they are using is not a true 31" dia. even though the tire says it is a 31-10.50 15 the actual dia differs depending on the manufacturer. i don't know if you can change the constant to reflect actual tire dia or there is preset constants in the psom

 

Ok getting closer. I found out the revs is 594. now i see that you multiply this by .0135 for 15 inch rima a .015 for 16 inch rims but what about 16.5's????
Thanks for all your help.
Scott

 

if you have the revs/mile and the rabs is 108, it doens't matter what the rim size is. that's calculated in your revs by the manufacturer. 108 x 594 = 64192 divided by 8000 = 8.02 constant

 

oh by the way the 108 is for for the small rear end- you may have a different rear in the 250 which would change the rabs number. look on your rear end there is a tag from the factory which should indicate how many teeth your rabs has.

 

 

ok I figurd it out. the tech artical is a little off because it says to multiply the number of revs by .0135 for 15" rims and .015 for 16"rims. the rim size doesn't matter at all what it really is is the number of teath on the sending ring which has 108 teath on the small diff and 120 on the heavy duty. I figured mine out for the larger rear end with 35" bfg's the revs I got from the bfg web site says 594 and multiplying that by the.015 give me the code of 891. which worked perfactly. Hopefully this isn't more confusing and helps clearify things a bit.
Thanks for all your help
Scott

 

Scott, you're right it's the gear teeth and not the rim size. I wrote the article but goofed on that point. I'll post a corrected copy. Thanks for keeping me in line!

 

yea, it confused me at first since I have 16.5's on mine but I got it figured out comparing the chart that is posted with the article.

Thanks again for that I wish I had found it a long time ago. I think that my trany mounts are shot also, and it is probably because it has shifted so hard for so long. but i can now get new one's and know that it is correct.

Scott