math question
#2
#7
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
63360 in/mi / Pi*D = 697.85 rev/mile
and for the 31 = 650.58 rev/mile
and the speed doesn't matter
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.
#8
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.
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.
#10
Unregistered User
Posts: n/a
Originally Posted by zabeard
235-75-15 = 717 revs per mile at 45mph
31-10.5-15 = 677 revs per mile at 45mph
31-10.5-15 = 677 revs per mile at 45mph
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
#13
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.
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.
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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