psom recalibration
Post Fiend
Joined: Nov 2001
Posts: 14,947
Likes: 6
From: Topsfield, MA
psom recalibration
Click the tech articles link on the right, there is an article in there on how to do it.
Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds
.
Check out my Gallery for a look-see.
Justin - One Happy FTE Member
Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds
.Check out my Gallery for a look-see.
Justin - One Happy FTE Member
Posting Guru
Joined: Dec 1999
Posts: 1,903
Likes: 2
From: Copley, OH
psom recalibration
Use a constant of:
.0135 x tire revs per mile
Your stock tires were approx 720 revs/mile: constant = 9.72
Your new tires are approx. 745-755 revs/mile depending on brand. For most brands you can look this number up on their web sites or at the tire rack. Assuming it's 745 revs/mile your constant would be 10.06. At 755 it would be 10.19.
.0135 x tire revs per mile
Your stock tires were approx 720 revs/mile: constant = 9.72
Your new tires are approx. 745-755 revs/mile depending on brand. For most brands you can look this number up on their web sites or at the tire rack. Assuming it's 745 revs/mile your constant would be 10.06. At 755 it would be 10.19.
Senior User
Joined: Oct 2002
Posts: 199
Likes: 0
From: S.E. PA
psom recalibration
I did mine, of course I have 31x10.5's and my speedo is right on, based on the calculation in the article. Maybe check with your tire dealer to see how much difference there is or see if the manufacturer has a web site where you could post a question. I think your size tire is about the same as a 31x10.5 
Posting Guru
Joined: Dec 1999
Posts: 1,903
Likes: 2
From: Copley, OH
psom recalibration
It's not even close in size. A 275/60-15 is only a 28" approx. dia. tire.
The formula I posted is based on the data table for conversion constants in my Ford shop manual. They don't cover your tire size in the table, so I extracted the factor based on tabulated factors and revs/mile. All you need to do is go to your tire manufacturer's web site and look up the revs per mile.
FWIW the difference between 745 and 755 revs/mile is less than 1 MPH at 65, so even if you use the wrong constant, it won't much matter.
The formula I posted is based on the data table for conversion constants in my Ford shop manual. They don't cover your tire size in the table, so I extracted the factor based on tabulated factors and revs/mile. All you need to do is go to your tire manufacturer's web site and look up the revs per mile.
FWIW the difference between 745 and 755 revs/mile is less than 1 MPH at 65, so even if you use the wrong constant, it won't much matter.
Post Fiend
Joined: Nov 2002
Posts: 9,987
Likes: 7
From: Memphis, TN
psom recalibration
Frogman
Go to TireRack.com and look up your exact tire, then find the "Specifications" link that shows a table of all tire sizes in that model and find the manufacturer's listed revs per mile. Use that in the Ford formula to find you true correct exact conversion constant, which is:
ConvConst = RevsperMile x ToneRingTeeth / 8000
(The '87-current 8.8" rear end has a 108-tooth tone ring. The 10.25" has 120 teeth.)
Go to TireRack.com and look up your exact tire, then find the "Specifications" link that shows a table of all tire sizes in that model and find the manufacturer's listed revs per mile. Use that in the Ford formula to find you true correct exact conversion constant, which is:
ConvConst = RevsperMile x ToneRingTeeth / 8000
(The '87-current 8.8" rear end has a 108-tooth tone ring. The 10.25" has 120 teeth.)
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Post Fiend
Joined: Nov 2002
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From: Memphis, TN
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