Tire Circumference

[Lawrence Coppari]

I've been curious for a while about the circumference of tires. I have particularly been wondering how close calculations are to the tire curcumference. These have an effect on acceleration times due both to moment arm length and actual speeds. Tire pressure would matter somewhat as well.

I mounted two different tires, put chalk marks on the tire and ground and pushed the car forward. Distances were measured. Here are results:

Tire Size Measured Computed

225/60 - 16 80 inches 83.7
245/45 - 16 74.5 inches 77.5
225/50 - 16 ??????? 78.1

Thus if the 225/60 -16 tires gives the true speed of 100 mph, the 245/45 - 16 tire only is going 93.125 mph. Big difference. The computed column was done by the data in the tire designation.

[97XRAY]

I'm sure you realize that this technique is crude. Any error you MIGHT have made, however small is amplified by the conversion to mph.
I've been curious too, though - so thanks for the preliminary results. If we had a cool dyno set-up or rollers we could really pin down the difference.

[Bob's944]

There's no real need to find out the tire's circumference to calculate speedo deviation. Since circumference is the diameter times pie (3.1428...) , you only need to find the tire height (that is the diameter of the tire) of the rear tires to be able to calculate speedo deviation when knowing the factory height used. So current rear tire height divided by factory tire height will give you a percentage that can be directly related to speed. Any height smaller than factory will make the speedo register higher than actual speed, and of coarse then anything larger will produce higher speeds than the speedo registers.

As an example, my 85.5 factory tires are 215/60/15, with a tire height of 25.2". A popular replacement tire is a 225/50/15, with a height of 23.9". 23.9 divided by 25.2 equals 0.9484 or 94.84%. So any speedo reading would then be 94.84% of the actual speedo reading (about 5% off). This is a 2.5 mph deviation at 50 mph, a full 5 mph lower at 100 mph.

[Lawrence Coppari]

The error lessens if you do more than one revolution of the wheel. And if you use a plumb bob to make sure the lines (on tire and ground) are marked correctly directly below the center of the axle, where is the error? Distance is distance, revolutions are revolutions. Suppose I missed by a half of an inch in a roll of 2 revolutions. The error is only .30% in per revolution. That is less than .3 mph at 100 mph. Doesn't sound crude to me. It sounds simple, yet effective to me. Didn't use lasers to do it, but it is accurate as to relative indicated speed. The smaller tire propels the vehicle only about 93% the speed of the larger tire given equal speedo readings.

I have two sports cars from the late 80's vintage. Both speedometers are incorrect and overestimate the speed by close to the same amount.