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  #1  
Old 12-21-2007
04 EDGE
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math question

can one of you smart guys tell me how many revolutions per mile a 235-75-15 does and also a 31-10.5-15 please and thank you.
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Old 12-21-2007
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Quote:
Originally Posted by 04 EDGE
can one of you smart guys tell me how many revolutions per mile a 235-75-15 does and also a 31-10.5-15 please and thank you.

235-75-15 = 717 revs per mile at 45mph
31-10.5-15 = 677 revs per mile at 45mph
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Old 12-21-2007
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Quote:
Originally Posted by zabeard
235-75-15 = 717 revs per mile at 45mph
31-10.5-15 = 677 revs per mile at 45mph
how u do that?!?!?
lol
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Old 12-21-2007
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you can go to tire rack... or use a converter and find the overall height then find the circumference, convert to feet and multiply by a mile.
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Old 12-21-2007
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also BFG has it on their site for every tire they sell
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Old 12-21-2007
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a 235/75 15 tire is 28.9" in diameter

63360 in/mi / Pi*D = 697.85 rev/mile

and for the 31 = 650.58 rev/mile

and the speed doesn't matter
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Old 12-21-2007
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Quote:
Originally Posted by RanJerEDGE
a 235/75 15 tire is 28.9" in diameter

63360 in/mi / Pi*D = 697.85 rev/mile

and for the 31 = 650.58 rev/mile

and the speed doesn't matter
That's the theoretical revs/mile. The true revs/mile is usually different because actual computed circumference is not necessarily the same as rolling circumference. Also, most "31-inch" tires are actually a little smaller than the sidewall size designation might imply.

For the most accurate number, it is best to use the measured data from the manufacturer's website. BFG site lists 30.7" diameter and 677 revs/mile for the 31x10.50R15 A/T KO used on the LII and 677 is exactly the number that my LII had in the PCM from the factory.
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Old 12-21-2007
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The circumference of a tire is simply the diameter times PI.

A 31" tire has a circumference of 31 * 3.142 = 97.4 inches.

97.4 inches is 8.17 feet more or less. A mile is 5280 feet so the number of revolutions is 5280 / 8.17 = 646. It's not really speed dependent in any great measure though tires do "expand" due to centrifugal force.

You can get more accurate measurements by substituting the exact diameter from manufacturers specs. A 31" tire for example, might only be 30.4 inches in diameter.

As far as metric sizes, I have a shortcut to tire height.

Take the first two numbers of the size and multiply them together. Then divide by 1270. Take that result and add the third number (rim size) and you have diameter.

In your example, a 235-75-15 would be done like this:

(235 * 75) = 17625
17625 / 1270 = 13.88
13.88 + 15 = 28.88 (or about a 29" tire)

I know that seems like a lot, but it's how it's done. I've reduced the method to convert the metric sizes by basically getting rid of all the brough-ha-ha about aspect ratio and metric to english conversion by consolidating the math.
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Old 12-21-2007
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Quote:
Originally Posted by Redneckstone
also BFG has it on their site for every tire they sell
thats what i did
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  #10  
Old 12-21-2007
04 EDGE
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Quote:
Originally Posted by zabeard
235-75-15 = 717 revs per mile at 45mph
31-10.5-15 = 677 revs per mile at 45mph
thank you sir you are my hero!

i'm still looking for that cover BTW.

i had my PCM re flashed yesterday and the best ford could give me was 677. so my speedo is still a little off.

at first i didnt believe them that they did anything at all to the truck but i guess they did. my matco scanner is telling me that the PCM is calibrated to 677 RPM's
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Old 01-18-2008
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where did you have it flashed? i need mine done to 631
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Old 01-18-2008
04 EDGE
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i had is done at (gasp gasp gasp) north brothers.

the best they could give me was 677 rev's per mile. they also said they reflashed my cluster for 4x4 but they didn't. i am waiting on a refund check per the owner doug north.
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Old 01-18-2008
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Quote:
Originally Posted by 04 EDGE
i had is done at (gasp gasp gasp) north brothers.

the best they could give me was 677 rev's per mile. they also said they reflashed my cluster for 4x4 but they didn't. i am waiting on a refund check per the owner doug north.
hahahaha I AM SORRY, i wouldnt wish there service dept on my worst enemy, I worked there for several months ya screw that... and have fun with getting money back form Doug that would be a once and a life time....


just FYI if ANYONE goes there for oil changes STOP NOW.. per DOUG they where suppost to put 4.5qt in if it called for 5... and so on.. because the extra would save them money... ya um ya no
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Old 01-19-2008
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When i worked at tom holzer we were told to put 5qts MAX in every gasoline engine. Wait a minute....the 3 valve 5.4 takes 7.25......not good
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