Tire revolutions/mile

Try this site, http://www.miata.net/garage/tirecalc.html

lightblade said:

Thanks!

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No problem sir, good luck and keep us up to date on your speedo.

Most of the 285/35-18s on tirerack are showing between 801-812 revs per mile. Your stock tires would have been 768 for the 17" and 759 for the 18". I don't know which you had. You know that your X-cal has them in incriments of 5 revs per mile for adjusting?

See, there's an interesting thing about tires and speedo "accuracy"... Our speedo's are never accurate. Brand new, 285/35/18's work out to about 25.85 inches in diameter. Thats assuming its measured at full tread depth, which could be an incorrect assumption, but its somewhat trivial in what I'm about to go through...

So brand new, we are looking at about 780 revs/mile. (I realize I'm using the theoretical calulations, and actual numbers may be slightly off, as you can see from the revs posted previously - it doesn't matter though). Since brand new, tread depth is about 10/32 and the tires are "worn out" at 2/32, that means you lose 1/4 inch of tread depth. Overall, the diameter of the tire drops by 1/2 inch, or 25.35 inches. This works out to about 796 rev/mile.

You are making another 16 revs for every mile you travel. Assuming calibration to brand new tires, when your tires are worn, every 48.75 miles you travel will show up with an extra mile on your odometer. This means your speedometer will be off by 2%, which isn't much, but highway speeds, thats around 1.5 mph.

This shows up more as the tire gets smaller, but it won't be much more significant on any tires sizes we are running on our mustangs.

Just an interesting thought. ***DISCLAIMER*** I have no idea how tire pressure and the compression of tires with weight on them effects this.

I got 2,451 revolutions in a mile
2,499 for 0.5" wear.

extra 48 Revolutions

48 revs = 1216" extra Wheel must turn during a mile

every 52 miles = 1 extra mile

1.9% @ 55mph = 1mph

Which isn't much at all. Losing a half an inch of tire diameter and still only losing 1mph.

mackey said:

Which isn't much at all. Losing a half an inch of tire diameter and still only losing 1mph.

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That was my point... but if you are getting 2500 revs/mile, that means you are running an 8 inch diameter tire! Don't forget about pi - it doesn't change the results we are going for since its a constant and you left it out in both cases

Another way to look at it is by assuming your car averages half tire tread. Reasonable assumption. That means ballpark, your odo will be adding 1 mile for every 100 you travel. Over the life of you owning the car - we'll say 36k miles just for our purposes. Your car would actually show about 36,360 miles. Try arguing that next time you want a warranty repair at 36,200 miles

stangdude2000 said:

we'll say 36k miles just for our purposes. Your car would actually show about 36,360 miles. Try arguing that next time you want a warranty repair at 36,200 miles

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If it is still on original tires and it works to your favor...why not!!!!!