opps 
Speedometer Recal
Tuned
Joined: Apr 2000
Posts: 393
Likes: 3
From: Merrill
Speedometer Recal
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
Thanks for the help
New User
Joined: Mar 2002
Posts: 15
Likes: 0
Speedometer Recal
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
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
Tuned
Joined: Apr 2000
Posts: 393
Likes: 3
From: Merrill
Speedometer Recal
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
Thanks for all your help.
Scott
New User
Joined: Mar 2002
Posts: 15
Likes: 0
Speedometer Recal
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.
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Tuned
Joined: Apr 2000
Posts: 393
Likes: 3
From: Merrill
Speedometer Recal
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
Thanks for all your help
Scott
Tuned
Joined: Apr 2000
Posts: 393
Likes: 3
From: Merrill
Speedometer Recal
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
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
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