Articles » Get the right rim/tire size combo for your car

One of the most popular mods for car enthusiasts is to take off those ugly OEM wheels and put on some (most of the time) larger wheels for better handling and visual purposes.

An important thing to keep in mind once you have your aftermarket rims picked out is to fit them with tires that are as close as possible to the original size of tire that came with your car. Why is this so important??

1) Your car's odometer is programmed to add up mileage based upon how many rotations your wheels make.

2) Your speedometer works the same way - it's all based off wheel rotation.

If your aftermarket wheels are drastically bigger or smaller than the original size you're going to throw off the accuracy of the odometer and speedo.

This article will teach you a simple equation that will make it easy for you to pick out the right sized tires for your new rims!

Lets go over some basics. One thing you will need to remember throughout this article is that ONE inch = 25.4 millimeters (mm). Aftermarket rim specifications are measured in mm so knowing this is important. Another important thing to remember is that if you need to find the OEM size of your car's tires it will be written on the sidewall of your tires, on a sticker right inside the driver's door, and it should be documented somewhere in the vehicles's owners manual.

I will be using a 2002 Civic Si as an example.

The OEM wheel is a 15" alloy wrapped in a 195/60/15 sized tire.
195 is the width of the tire in mm, 60 is the aspect ratio as a percentage of the tread width. So the sidewall is 60% of 195 mm or 117 mm which is about 4.6 inches. Finally, the last number, 15, is simply the size of the wheel the tire is going on. Easy enough.

Now lets throw some 18's on that Si! We need to find out the total diameter of the OEM tire and then find an aftermarket tire of about the same diameter.

Here's how:

If the sidewall is 60% of 195 (195/60/15) you should divide 195 by 100 = 1.95. So one percent of of the tire tread is 1.95 mm. Multiply that by 60 and you have the sidewall size; 117 mm. Now multiply that by two = 234 mm. Write that number down because we'll be adding it to another number.

The second step is to take the third number, 15 in this case (the size of the wheel the tire is on) and multiply that by 25.4 (the number of mm in an inch). So 25.4 multiplied by 15 = 381mm. Now we add both numbers to get the total tire diameter in mm: 381mm + 234 mm = 615 mm OR 24.2 inches. In conclusion, the diameter of the OEM 2002 civic Si tire is 24.2". So the next step is to find an aftermarket tire with as close to the same diameter as possible.

Two tire sizes you might run into with an 18" wheel are 225/40/18 and 215/35/18 - it's all about finding the one that is closest to the OEM tire diameter that came with your car.

If you do the math you'll find that the 225/40/18 tire's overall diameter is 637.2 mm or about 25 inches. That's pretty close to the OEM tire size of 615mm/24.2 inches, right?

Lets look at the other tire size, the 215/35/18. After the math you'll find that the diameter in mm is 607.7 - about 23.9 inches. Both are close but the 215/35/18 would be the better tire to get - it's only 8 mm smaller than the OEM size...less than 1/3 of an inch smaller.