How to Calculate Tire Sizes

How would you determine the size of this tire?

Getting the tire size for your car is usually simple, since such data is in the owner's manual and sometimes even printed on a sticker inside the driver's side door. However, you may have specialized rims, or you may not have access to the data and need to start over from scratch.

Things You'll Need

• Tire gauge (optional)
• Tape measure

Instructions

1. Check an existing tire to make sure it is properly inflated. You can do this with a tire gauge, but if you don't know what the proper pressure is, that reading is useless. In that case, push against the sidewall of the tire with your thumb as hard as you can. If there is a little give, but not much, the inflation is good enough for a measurement.

2. Take a tape measure and determine how wide the tire is across the top, from sidewall to sidewall. Use the metric system, as most passenger tire sizes are noted in mm. A typical example for a passenger car is 225 mm.

3. Measure the sidewall next, and then determine that measurement as a percentage of the number from step 2. Passenger tire sizes are noted as tire width/sidewall percentage of tire width. For example, 225/50, or 225 mm wide and 50 percent for the sidewall is a typical measurement. If this tire has a width of 112.5 mm (half of 225), it is a 225/50.

4. Add "LT" to the designation if you have an SUV or pick-up truck. The tire sizes for these larger vehicles start with an "LT" for "light truck."

References

• Tire Size Metrics
• Wheel and Tire 101