I have changed the tire size on my truck from p235/75r15
to p275/60r15 the conversion constant is set at 972(factory setting)
what do I need to change it to?

 

Click the tech articles link on the right, there is an article in there on how to do it.


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Justin - One Happy FTE Member

 

I have read probably 50 responses from people and from sites and they all aggree on how to do it they do not agree on the tire constant.

 

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.

 

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

 

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.

 

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.)