Tire revs per mile

[Steve D.]

When I calculate revs per mile I get a number that is consistently 3%below published numbers. For example 215-70-R15 with diameter 26.8". I divide 63,360 (number of inches in a mile) by 84.195 (tire circumference in inches, 26.8*pi) and get 752.5 revs per mile. But all the published specs I've seen show about 775 revs per mile. What am I missing in the calculation?

Also, I have seen revs per mile given at a particular speed (45 mph). What does the speed have to do with it, since revs per mile doesn't have an element of time in it?

Steve

[Mike Cobine]

Re: Tire revs per mile

It is real simple. Tires are not solid objects, they flex, and so the diameter you measure is not necessarily the one used for the calculations.

The most accurate way of getting the circumference of a tire is to have it ON THE CAR and mark the tire and that point on the ground, then roll the car until the mark hits the ground again. Measure between the two points.

You will find that this measurement should fit in the right number of revs per mile.

The speed is a factor in that a tire "grows" with speed. The tread stretches outward the faster you go. The next time you get a chance, what a drag race with dragsters and notice how the rear slicks change from short fat tires to very tall skinny tires as the tire speed increases when they do a burn-out.

[Steve D.]

Re: Tire revs per mile

Mike

I've been using the published "Overall Diameter" spec. If the effective diameter gets larger with speed, the revs per mile should go down. My calculated numbers for several tire sizes is 3% low on each size. Since the difference is constant, I thought that there might be an assumed constant in the equation somewhere.

Thanks.

Steve

[Duke W.]

Re: Tire revs per mile

I explained this in detail in The Corvette Restorer last year. Simply put and as stated previously, tires are not rigid bodies, and because of distortion when loaded, they have greater revs per mile than the rigid body calculation from the overall diameter. Actual revs per mile varies with inflation pressure, load, speed and tread wear. For example, revs per mile increases about 2 percent between new and the wear bars. Also, manufacturing tolerance can cause variance from published data by up to two percent.

At typical loads and pressures the published revs/mile are usually quite accurate and should be used in speed-rev calcuations. Radial tires, particularly higher speed rated nylon cap belted radials, grow very little with maximum speed - no more than about one percent.

Duke