formula to figure actual milage

I have 3.55 gears and went from tire size 225 to 31" did not change gears my speedo is still off so when I am going 65 it says a little under 60mph. I want to know how many miles I have on my current tires I got them when the odometer said 78,000mi and it now says 96,500. I had a ratio wrote down and lost it thanks. I get lots of questions about my TRXUS milage and don't know what to tell them I think I might be the first to wear a set of these down.
DIG IT!

well it depends on what tires came stock on the vehicle and what speedo gear it has in it but a rough estimate is that you have 98,500 miles on it. This is an 11% correction factor for the tire size. I used the correction factor for my '87 which came with 215's and has 31's on it now.

BUCKYXJ said:

I have 3.55 gears and went from tire size 225 to 31" did not change gears my speedo is still off so when I am going 65 it says a little under 60mph. I want to know how many miles I have on my current tires I got them when the odometer said 78,000mi and it now says 96,500. I had a ratio wrote down and lost it thanks. I get lots of questions about my TRXUS milage and don't know what to tell them I think I might be the first to wear a set of these down.
DIG IT!

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According to your odometer, you've put 18,500 miles on the tires. Because of the the larger 31" tires, you have put more miles on the tires than the indicated mileage.

How big are the 225's:

225 / 25.4 = 8.86 inches wide
8.86" * .78 = 6.91" sidewall (guessing this is a 78 series tire)
15" + (6.91 * 2) = 28.8"

What's the difference:
31" / 28.8" = 1.076 or 7.6% increase in size

So the actual mileage is:
18,500 * 1.076 = 19,906 actual miles

And for the speedometer:
60MPH * 1.076 = 64.56MPH

The number that really counts is revolutions per mile, and most of the tire manufacturers publish that for their tires. Not all tires of the same nominal size have the same revolutions per mile, because things like sidewall stiffness may make one tire effectively taller than another.

I pulled some figures from various manufacturers awhile ago and made up a chart. For a stock 225/75R15 I show 730 revolutions per mile. For a 31x10.50R15 I show 680 revolutions per mile.

The correction factor is the difference divided by the original number, or 50/730 = 6.85%

This actually surprised me, because I expected a bigger difference. I ran my '88 for years on 215s, and when I went to 30x9.50s I calculated the difference at 6% -- and it would have been 10% going to 31s. I guess the 225s are that much larger than the 215s. Who would'a thunk it?

BUCKYXJ said:

I am going 65 it says a little under 60mph. I want to know how many miles I have on my current tires I got them when the odometer said 78,000mi and it now says 96,500.

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If these figures you've given are correct -


Ratio of actual mileage to indicated mileage:

65 / 59.75 = 1.0878661 . . .


Indicated miles traveled:

96,500 - 78,000 = 18,500


Actual miles traveled:

1.0878661 * 18,500 = 20,125.5230 . . .

There's no such thing as a 78-profile tire, is it? It's either 215/70-15 or 215/75-15. I'm guessing 75, so here's my ratio: 1,1192628751335894399352993847665 You don't need all those decimals, of course...

However, few 31" tires are truly 31", they usually cheat you about 1/2 inch of rubber. The tire is also sueezed flat when the car is weighing on it, and different rim widths and air pressure can cause the tire to grow or shrink in diameter.

The best (and really only correct way) to measure a wheel's rolling lengt, is to do this:

Put your car on flat ground. With a chalk or something, draw a line from the tire and down on the ground, leaving a line on the ground as well.

Drive your car until the mark on the tire hits the ground again, after one revolution. Draw another line down to the ground from this line. Measure the distance between the lines. Now you have a quite accurate measure of how far your tire's actually rolling with current load and air pressure. To get it all correct you should do this with your 215's on as well! I totally understand if it's too much hassle...

Amund2 said:

There's no such thing as a 78-profile tire, is it? It's either 215/70-15 or 215/75-15. I'm guessing 75, so here's my ratio: 1,1192628751335894399352993847665 You don't need all those decimals, of course...

However, few 31" tires are truly 31", they usually cheat you about 1/2 inch of rubber. The tire is also sueezed flat when the car is weighing on it, and different rim widths and air pressure can cause the tire to grow or shrink in diameter.

The best (and really only correct way) to measure a wheel's rolling lengt, is to do this:

Put your car on flat ground. With a chalk or something, draw a line from the tire and down on the ground, leaving a line on the ground as well.

Drive your car until the mark on the tire hits the ground again, after one revolution. Draw another line down to the ground from this line. Measure the distance between the lines. Now you have a quite accurate measure of how far your tire's actually rolling with current load and air pressure. To get it all correct you should do this with your 215's on as well! I totally understand if it's too much hassle...

Click to expand...

78 series tires were available back when tire companies were transitioning between the old "alpha" sizes and the new.
remember "G60 15" There was also a "78" aspect ratio. This shows my age Your .75 is a much better guess.

The best way is to take the mfg's rev's per mile like Eagle did. That will be very close to a calculation based on rolling radius, corrected for average tread wear. (I'm guessing).

Using the chalk mark and roll method isn't that accurate. The tread squirm will allow the tread to compress at the point where the front of the tire is approaching the ground. The net result is based on rolling radius.

I don't correct for the disparity in the actual size of various 31" tires because the 215's play by the same rules.

MaXJohnson and I must be close in age -- I also remember some G78 tires that came on my mother's 1970 station wagon. I don't think that profile is currently available, and if it is I doubt it is offered in off-road or A/T type tires that we'd be putting on an XJ.

The OEM tires on the XJ were all 75 series (profile ratio), with the exception of the 2000 and 2001 Classic and Limited. Those came with 16" rims, so they used a tire with the same cross section (225) but a 70 series profile to keep the overall diameter the same.

MaXJohnson said:

Using the chalk mark and roll method isn't that accurate. The tread squirm will allow the tread to compress at the point where the front of the tire is approaching the ground. The net result is based on rolling radius.

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I was thinking about this on the way home from work. Amund2's method is likely as accurate as any, although multiple rotations would increase the accuracy even more.

Ya gotta keep your eye on this MaX dude or the bullshi+ will start flying.

Yes, I'm guessing "my" (it's not really my invention ) method is quite accurate, the rubber will squeeze, but only upwards, and when the chalkmark is straight down it will be totally accurate, and the distance the car actually has rolled is not lying. You could increase the accuracy by measuring more than one revolution, I'm guessing two or three will be optimal.