The Importance of Tire Diameter
Have you ever stopped to consider how your car’s speedometer & odometer know how fast you’re going or how far you’ve gone? Even in this day of high-tech, it’s all done the old fashioned way – counting revolutions.
The revolutions being counted is coming from the output your transmission, such as counting driveshaft turns. Car designers know the ratio of driveshaft to wheel turn and the size of tire that will be put on your car and can calculate how many revolutions the shaft will turn every mile.
There are two things that can be changed which will mess up the designers’ calculations: Changing the gears in the differential and changing tire size.
Here’s how changing tire size works. We’ll start with my Spider’s stock tires of 185/70R14, which we’ve computed to have a tire diameter of 24.20 inches. To get the circumference of the tire you multiply by pi (3.14159) and get 76.03” (6 1/3’). So the Spider travels 6.33’ every time the wheel goes around.
There are 5,280 feet in a mile. By dividing that by 6.33’ we get 834.12. That’s how many revolutions the wheel turns in one mile.
Now let’s say I went to 190/65R14 tires, with a diameter of 23.72”. The circumference of this tire is 74.52” (6.21’). Multiply this by 834.12, and you get 5,179’, or 98% of a mile. Your odometer, then, records that you’ve traveled more miles than you actually do, which could be a warranty consideration.
Your speedometer will also be fast, which might be okay if you suddenly encounter a speed trap. Of course, if you went the other way with your tires (larger diameter than stock) everything is reversed and you’d be left swearing the cops radar gun was off.
There’s more – everything back of the wheels get affected. Since your wheels have to make more revolutions for every mile, do your ball bearings, differential, drive shaft, transmission, and of course, the engine. This means more wear and tear on all these parts. It also means lower gas mileage, both the perceived 2% reduction as well as a real reduction since you’ll probably drive 2% faster, and the faster you go the lower the actual gas mileage.
You don’t have to go through all of the computations shown here. Once you’ve computed your new tire diameter simply divide it by your stock diameter and you’ll get the same percentage. Just remember that if the number is low (rhymes with slow) your odometer & speedometer is fast.
