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Old 10-12-2016, 10:31 AM
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Can A Change In Tire Dia. Be Calculated Accurately??

Here's the deal. My truck came equipped with P-235 X 15 tires from the factory. I don't know the rolling circumference of that tire. After they were worn, (around 22,000 miles), I replaced them with 31 X 10.5 X 15 tires. Obviously these tires have a larger rolling circumference that has effected both odometer and speedometer accuracy. I have had that size tire on the truck ever since.

My truck now has 152,000 miles on it. Which means for the last 130,000 miles it has registered fewer miles, and a lower indicated MPH because of the larger diameter tires. Is there a way to calculate the difference, so I know with some degree of accuracy, how many actual miles my truck has on it?

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Old 10-12-2016, 10:41 AM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

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Originally Posted by billt View Post
Here's the deal. My truck came equipped with P-235 X 15 tires from the factory. I don't know the rolling circumference of that tire. After they were worn, (around 22,000 miles), I replaced them with 31 X 10.5 X 15 tires. Obviously these tires have a larger rolling circumference that has effected both odometer and speedometer accuracy. I have had that size tire on the truck ever since.

My truck now has 152,000 miles on it. Which means for the last 130,000 miles it has registered fewer miles, and a lower indicated MPH because of the larger diameter tires. Is there a way to calculate the difference, so I know with some degree of accuracy, how many actual miles my truck has on it?
Yes, but if you are concerned that much about accuracy, it would be a royal PITA.

You'd need the original circumference of the new tires plus the current circumference of those tires [tread wear] and from there it would be a guesstimate as to the rate of wear--but you'd be pretty close by presuming it is constant, or just do it without calculus by assuming a constant value for every 10K miles or similar increment where you take the current value and the original value and divvy it up.

Then calculate the difference between displayed mileage and actual mileage for each of those 10K [or 5k, pick a value] increments.

A tad easier with calculus, but you can accomplish the same thing with a reasonably small increment value using Newton approximation.

This does presume a lot, enough to pretty much make it an interesting math project, but a waste of time unless you regularly calibrated your mileage display on the vehicle and the tires as they wear, plus have accurately measured the circumference at various stages. The degree of accuracy will depend on your tolerance and beer ingestion during the project.
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Old 10-12-2016, 10:54 AM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

I'm not concerned about wear, because all tires wear about the same amount over the same mileage mounted on the same vehicle, with all other things considered equal. (I realize that circumference changes with wear). I'm more interested in calculating the difference in rolling circumference of the 2 sizes over 130,000 miles. That should give me enough information to go with.
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Old 10-12-2016, 10:58 AM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

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Originally Posted by billt View Post
I'm not concerned about wear, because all tires wear about the same amount over the same mileage mounted on the same vehicle, with all other things considered equal. (I realize that circumference changes with wear). I'm more interested in calculating the difference in rolling circumference of the 2 sizes over 130,000 miles. That should give me enough information to go with.
If you assume the original tires and displayed mileage are correct, the correction for the larger tires is just the ratio of the [larger] circumference and the displayed miles for them.

Don't even need pi, unless you like some with your coffee.
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Old 10-12-2016, 11:38 AM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

This tire comparer shows circumference and percentage differences between the speedometer. I would think the percentage difference will apply to mileage also, or close enough for what you are trying to do.

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Old 10-12-2016, 11:39 AM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

Quote:
Originally Posted by billt View Post
Here's the deal. My truck came equipped with P-235 X 15 tires from the factory. I don't know the rolling circumference of that tire. After they were worn, (around 22,000 miles), I replaced them with 31 X 10.5 X 15 tires. Obviously these tires have a larger rolling circumference that has effected both odometer and speedometer accuracy. I have had that size tire on the truck ever since.

My truck now has 152,000 miles on it. Which means for the last 130,000 miles it has registered fewer miles, and a lower indicated MPH because of the larger diameter tires. Is there a way to calculate the difference, so I know with some degree of accuracy, how many actual miles my truck has on it?

Try this link below...plug in your info and once it computes scroll down on that page and it will tell you the difference revolutions and then you should be able to do the math to get a pretty close reading...good luck....I hate math

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Old 10-12-2016, 11:42 AM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

Quote:
Originally Posted by Governor View Post
This tire comparer shows circumference and percentage differences between the speedometer. I would think the percentage difference will apply to mileage also, or close enough for what you are trying to do.

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com


That's funny...your post beat mine by 1 minute
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Old 10-12-2016, 11:46 AM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

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Originally Posted by WK2OverlandII View Post
That's funny...your post beat mine by 1 minute
Great minds....
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  #9  
Old 10-12-2016, 06:19 PM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

Quote:
Originally Posted by WK2OverlandII View Post
Try this link below...plug in your info and once it computes scroll down on that page and it will tell you the difference revolutions and then you should be able to do the math to get a pretty close reading...good luck....I hate math

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That's excellent! Thank you.
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Old 10-12-2016, 06:59 PM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

You're missing part of the size of your original tire...the aspect ratio. It makes a huge difference on the diameter (which is all you really need for your calculations...the diameter of both tires)....A 235/50-15 has a much shorter sidewall, and therefore smaller overall diameter than a 235/75-15, for example.
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Old 10-12-2016, 07:23 PM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

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Originally Posted by DriverDave View Post
You're missing part of the size of your original tire...the aspect ratio. It makes a huge difference on the diameter (which is all you really need for your calculations...the diameter of both tires)....A 235/50-15 has a much shorter sidewall, and therefore smaller overall diameter than a 235/75-15, for example.
They were 235 / 75 / R15. I'm currently running 31 X 10.5 X 15.
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Old 10-12-2016, 07:36 PM
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Re: Can A Change In Tire Dia. Be Calculated Accurately??

A quick search shows your original tires were 28.9" dia (new) and the new ones are, of course, 31" (new). It will take a bit of searching/calculating to sort out the mileage difference between the 2, but you're on your own for that ;-)
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