Things You'll Need:
- Tape measure
- Pencil
- Paper
- Tire de-rimmer
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Step 1
Measure the size of the rims you want to use with the car's tires. The overall diameter of the rims (in mm) is the pertinent number. Most often, this information is included in the car's owner's manual, but if you've made adjustments since then or want to check for yourself, you should measure.
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Step 2
Check the code--the string of numbers and letters--on the tires you intend to use. The first three numbers (after the letter) determines the tire's width in mm, while the number after the slash represents the tire's aspect ratio (the height of the tire from the bead to the top of the tread, described as a percentage of the tire's width). The last set of numbers state the wheel's rim diameter, in inches.
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Step 3
Determine the rim width recommendation for your new tires. They need to fit the rims to be safe. You can get this information from the owner's manual, or by contacting a local dealer or the car manufacturer and requesting the information.
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Step 4
Calculate the overall tire diameter, as recommended by the manufacturer. To do this, multiply the sum of a tire's width and aspect ratio by two, then add the product to the wheel diameter in mm. You want to maintain this overall diameter in any tires you modify for your car.
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Step 5
Calculate the diameter of the new tires. You can choose to modify tires with a lower aspect ratio, lower width, higher aspect ratio or higher width, so long as the overall tire diameter matches that of the original tires. Use an online tire and rim upgrade calculator if you want to take a shortcut (see Resources below).
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Step 6
Remove the old tire from the rim using a tire de-rimmer.
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Step 7
Check the rim carefully for any damage.
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Step 8
Remount the new tire onto the rim and inflate it to proper specifications.
