How to Change Your Tire Size Safely

Beginner · 30 minutes to plan

Changing tire size — plus-sizing for looks, going taller for clearance, or just finding a substitute when your exact size is out of stock — is easy to get wrong in ways that throw off your speedometer, rub the fenders, or confuse the traction control. The key number is overall diameter. As long as a new size keeps the diameter within about 3% of stock, your speedometer stays accurate and the systems that depend on it behave. This guide shows how to read the code, do the math, and check fitment before you buy.

What you'll need

• Your current tire size code (on the sidewall, e.g. 225/45R17)
• The tire size calculator to compare diameters
• The candidate new size you're considering
• Your vehicle's tire placard (driver's door jamb) for the factory size and pressures
• A tape measure to check wheel-well and suspension clearance

Steps

1. 1

   Read your current tire code

   Find the size on the sidewall: width in millimeters, aspect ratio (sidewall height as a percent of width), and rim diameter in inches — for example 225/45R17. The placard in the door jamb confirms the factory size.

2. 2

   Calculate your stock diameter

   Enter your current size in the calculator to get the overall diameter. This is your baseline — every comparison is measured against it.

3. 3

   Calculate the new size's diameter

   Enter the candidate size and read its diameter. Note the difference from stock; that change is what affects your speedometer, gearing, and clearance.

4. 4

   Keep within ~3% of stock diameter

   Aim to stay within about 3% of the original diameter. Beyond that, your speedometer and odometer drift noticeably and traction/stability systems read the wrong speed.

5. 5

   Check width and offset fitment

   Confirm the new width fits your wheel's rim width range, and that the tire clears the suspension and fender at full lock and full compression. A wider or taller tire can rub even with a similar diameter.

6. 6

   Verify load and speed ratings

   Make sure the new tire's load index and speed rating meet or exceed the factory spec on your placard. Under-rating either is a safety problem, not just a performance one.

7. 7

   Adjust pressure and recheck the speedometer

   Set the inflation to the placard pressure for your load, then verify the speedometer against a GPS app on a drive. A small steady error confirms how far the new diameter shifted things.

Pro tips

• Stay within ~3% of stock diameter unless you're deliberately re-gearing or recalibrating — the calculator shows the diameter delta instantly.
• Plus-sizing (bigger rim, shorter sidewall) should keep the overall diameter roughly the same so handling stays predictable.
• Never drop below the factory load index. A tire that can't carry your vehicle's weight is unsafe regardless of how good it looks.
• A taller tire makes your speedometer read slower than you're actually going; a shorter one reads faster. Verify against GPS after a change.

FAQ

Ready to estimate materials? Use the Tire Size Calculator →