Changing Tire Size: Diameter, Load and Speedometer

The short answer

You can change tire size on many cars, but the tire makers' starting point is not to. USTMA says replacement tires should be the same size, load index and speed rating as recommended by the vehicle or tire manufacturer. Michelin adds that a change from the specified size should only be made with guidance from a trained tire professional.

That's not because a different size never works. It's because the size on your door placard was chosen to fit everything else on the car: the speedometer, the ABS and stability systems, the wheel wells and the load the car carries. Change the tire and you need to keep all of those working.

The simplest change is between sizes the vehicle maker already approved. Michelin's guidance is to use the same size designation or approved options recommended by the vehicle or tire manufacturer, and some owner's manuals list more than one size. Anything beyond that list is where the checks below come in.

Here's what has to stay the same, and what to check.

1. Keep the overall diameter close

Overall diameter is how tall the tire stands, rim plus two sidewalls. It's the number that matters most when you change size, because the car's systems are calibrated to it.

Michelin says altering overall diameter affects speedometer accuracy, the clearance between the tire and the wheel well, and potentially how the ABS and stability control systems behave.

You can work it out from the size. Our guide on how to read tire size walks through the math. Using 225/65R17 as the original, about 28.5 inches tall:

The last row shows why sizes aren't interchangeable just because the width matches.

How close is close enough? Fitment guides commonly work to about 3% of the original diameter. Treat that as a guideline, not an approval. The car's maker and a fitment check have the final say.

2. Plus sizing: bigger wheel, same height

Most size changes are plus sizing: a larger wheel with a lower-profile tire.

Michelin explains it with a simple example: going from a 14-inch to a 15-inch wheel is plus 1. The tire's aspect ratio drops as the wheel grows, so the overall diameter stays about where it was. Michelin also describes the other route: keeping the same wheel and going wider, such as P195/75R14 to P215/65R14.

The right width and aspect ratio depend on the exact sizes, so compare the overall diameter of the old and new size before you buy. The plus-size calculator does the math for any two sizes.

Plus sizing usually means new wheels too. The width, offset and bolt pattern of those wheels have to suit the car; our guide to wheel offset and backspacing explains what changes.

3. Load index and speed rating: never lower

A different size doesn't lower what the tire has to carry.

• Load index. USTMA and Michelin both say not to choose a tire with less load-carrying capacity than the original. Match or exceed the load index on the door placard. If the new size carries an XL marking, our XL vs SL guide explains what it means for pressure.
• Speed rating. Match or exceed the speed rating the vehicle maker specifies.

The load and speed checker compares a new tire's ratings against your current ones.

4. The speedometer effect

Your speedometer counts wheel turns and assumes the original tire size. The result:

• A taller tire covers more ground per turn, so the speedometer reads slower than your actual speed.
• A shorter tire covers less ground per turn, so it reads faster.

Using the table above, a 1.8% taller tire means that when the speedometer shows 60 mph, you're doing about 61. The 5.4% shorter size would have you at about 57 with the needle on 60. The odometer is off by the same proportion.

The calculator below shows what your speedometer would read at 30, 45, 60 and 70 mph for any two sizes. It's also on its own page as the plus-size calculator.

5. Clearance and everything around the tire

A size that's right on paper still has to physically fit. Check:

• Clearance to the fender, wheel well and suspension. Michelin names wheel-well clearance as one of the things a diameter change affects.
• The wheel. The tire has to suit the width of the wheel it goes on, and a new wheel's offset changes where the tire sits.
• Front and rear. On a staggered car, the front and rear sizes differ, so a size change involves both axles. Our staggered tires guide explains the setup.

This is the part a fitment check exists for.

Before you order a new size

1. Find your current size, load index and speed rating on the door placard. The Tire Size Finder reads the sticker for you.
2. Compare the new size's overall diameter in the plus-size calculator.
3. Confirm the new tire meets or exceeds the load index and speed rating.
4. Have the fitment confirmed for your car and wheels before you order.

Then shop tires by size or browse wheels. Extreme Tires installs at the Sunrise shop or by mobile van in Miami-Dade, Broward and Palm Beach counties. Anywhere else in the 48 contiguous states and DC, we ship to your address for a local installer. See how installation works, or call (954) 773-1896 to check a size change before you buy.