math calc for new tires

Hello, can someone tell me how to calculate the speed change on my car since I put new wheels and tires on the car. I still have the old wheels and tires that I can measure against, I am curious how off my speedometer is as well as how much my odometer will be off as well.


I’m not an automotive expert, but if I understand how speedometers work, it should be simple.

If you have both sets of tires available, the simplest thing to do will be to measure the circumference of both. Any percentage difference will be the amount your speedo has changed.

For example, let’s say your old tires were 75 inches around (just pulling numbers out of the air) and your new tires are 70 inches. That’s a reduction of 5 inches, or about 7 percent. Your speedometer would now read about 7 percent slow, so if it said 65 miles per hour, it would really be about 60.5 mph.

I think your odometer would be off by the same percentage.


Actually if you put on smaller tires than normal as in the above example, the speedometer will read faster than normal, not slower, for a given actual speed.

If the new tires are of a different brand/model you may not get a accurate number by doing a static measurement. It seems that tires “grow” in
circumference as the car speeds up. If your old tires grew by a greater or lesser degree than your new onesit can throw you calcualtions off.
The best way is to go to the auto club (who sometimes have clinics and will check this for free) or swing by a speedo shop who will put your car on a dyno and give you a speed by speed comparsion between actual and indicated speed.

As RJKUgly says, the factor is the ratio of the circumferences, which is the same as the ratio of the radia. Since we assume that the speedometer (& odometer) were calibrated for the original size of tires (radius O), then replacing these with new tires of radius N means the change in readings will be O/N. As an extreme example, imagine your new tires are twice the size of the old ones. Then in one revolution of the new tires your car will move exactly as far as it would for two revolutions of the old tires. Thus, for any speed, the speedometer would read exactly half the true value.

You guys have it, it is the ratio of the circumferences.

Unless you radically change the tire compound or profile (say, from 70 series to 45 series), they should likely ‘grow’ a similar ratio at speed. Or, at least little enough that the error will be small.

Yeah, my example was correct, but saying “7 percent slow” was definitely bad wording.

In my example, I said the speedometer would read 65 mph when the true speed would be about 60.5 mph. This is correct. But the “7 percent slow” should really have been “7 percent high”. The true speed would be about 7 percent lower than the speedo reading (as my actual figures indicated), not the other way around.

Sorry for any confusion.


Drive by a speed monitor thing.