Tire size calculator
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Posting Guru
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From: Juneau, ALASKA
Tire size calculator
Postmaster
Joined: Apr 2006
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From: CA Bay Area
This doesn't show actual diameter, but the theoretical one. I found 2 different models of tires, coming from the same manufacturer in the same nominal size having like 12% difference in rolling diameter.
The only real thing is to find rpm (rounds per mile) for the actual tire on manufacturer data sheet.
The only real thing is to find rpm (rounds per mile) for the actual tire on manufacturer data sheet.
Fleet Mechanic
Joined: Jul 2003
Posts: 1,815
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From: Houston
By rolling diameter do you mean with the tire on the ground as opposed to off the ground? That is what I have heard of as rolling diameter. Not sure how accurate.
However...12%? That is huge! on a 30" tall tire that is over 3.5" of difference. That is no longer a 30" tire...that is a 33". What tire manufacturer are you referring to?
Postmaster
Joined: Apr 2006
Posts: 3,524
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From: CA Bay Area
Measuring tire "working" diameter is tricky business.
Tires flex with inflation and load, meaning with bigger load the same tire is going to have smaller rolling diameter. Than at speeds the centrifugal forces change the rolling diameter as well.
That didn't matter in the past, but now ABS, traction and ESP systems are having speed sensors at each wheel and when one tire spins with different rpm, it will take it as lost traction. Usually the systems give you 3% tolerance before kicking warning lights.
So if you are matching different tires, you have to go to the tire data sheet and read the rpm. The manufacturer doesn't matter >>> all of them manufacture tires with pretty loose tolerances.
Tires flex with inflation and load, meaning with bigger load the same tire is going to have smaller rolling diameter. Than at speeds the centrifugal forces change the rolling diameter as well.
That didn't matter in the past, but now ABS, traction and ESP systems are having speed sensors at each wheel and when one tire spins with different rpm, it will take it as lost traction. Usually the systems give you 3% tolerance before kicking warning lights.
So if you are matching different tires, you have to go to the tire data sheet and read the rpm. The manufacturer doesn't matter >>> all of them manufacture tires with pretty loose tolerances.
Fleet Mechanic
Joined: Jul 2003
Posts: 1,815
Likes: 24
From: Houston
Measuring tire "working" diameter is tricky business.
Tires flex with inflation and load, meaning with bigger load the same tire is going to have smaller rolling diameter. Than at speeds the centrifugal forces change the rolling diameter as well.
That didn't matter in the past, but now ABS, traction and ESP systems are having speed sensors at each wheel and when one tire spins with different rpm, it will take it as lost traction. Usually the systems give you 3% tolerance before kicking warning lights.
So if you are matching different tires, you have to go to the tire data sheet and read the rpm. The manufacturer doesn't matter >>> all of them manufacture tires with pretty loose tolerances.
Tires flex with inflation and load, meaning with bigger load the same tire is going to have smaller rolling diameter. Than at speeds the centrifugal forces change the rolling diameter as well.
That didn't matter in the past, but now ABS, traction and ESP systems are having speed sensors at each wheel and when one tire spins with different rpm, it will take it as lost traction. Usually the systems give you 3% tolerance before kicking warning lights.
So if you are matching different tires, you have to go to the tire data sheet and read the rpm. The manufacturer doesn't matter >>> all of them manufacture tires with pretty loose tolerances.
I'm well aware of the havoc tires can play on newer vehicles. Take a look up in the Expedition forum and you will see problems with worn different tread depths (same model/size tire) on the Expedition AWD models. You really have to watch what you do or sensors will quickly let you know they didn't like it.
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Brakes, Steering, Suspension, Tires, & Wheels
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