What Tire Size Do I Need for Bigger Wheels?

Pick a tire with a lower aspect ratio, and often a slightly wider section, so that the overall diameter stays close to the original. For example, a car on 205/55R16 (24.88 in) moving to 17-inch wheels can use 225/45R17 (24.97 in, +0.38%), and on 18-inch wheels 225/40R18 (25.09 in, +0.84%).

This is called plus sizing: +1 means a wheel one inch larger, +2 means two inches larger. The wheel grows, the sidewall gets shorter, and the total height stays about the same.

Why keep the overall diameter the same?

The speedometer, the odometer and the wheel-speed sensors used by ABS and traction control are set for the original tire diameter. A taller or shorter tire changes all of them. A tire that is much taller can also touch the fender or the suspension.

A common industry guideline is to stay within about 3% of the original diameter. It is a guideline for comparing sizes and speedometer error, not a law and not a safety limit. Closer is better, and AWD vehicles often need much closer matching (see can you mix tire sizes on an AWD vehicle).

How do you calculate the new tire size?

1. Find the original overall diameter (mm) = rim x 25.4 + 2 x width x aspect / 100.
2. Find the sidewall height you need = (original diameter - new rim x 25.4) / 2.
3. Find the aspect ratio = needed sidewall / width x 100. Round to the nearest multiple of 5.
4. Check the result with the diameter formula and compare it with the original.

Example: 205/55R16 to 18-inch wheels (+2)

1. Original diameter: 16 x 25.4 + 2 x 205 x 0.55 = 406.4 + 225.5 = 631.9 mm (24.88 in).
2. Sidewall needed: (631.9 - 18 x 25.4) / 2 = (631.9 - 457.2) / 2 = 87.35 mm.
3. With a 225 mm width: 87.35 / 225 = 38.8%, rounded to 40. With 215 mm: 87.35 / 215 = 40.6%, rounded to 40.
4. 225/40R18: 457.2 + 2 x 225 x 0.40 = 637.2 mm, +0.84%. 215/40R18: 457.2 + 172.0 = 629.2 mm, -0.43%.

Both are close. At +0.84%, a true 60 mph shows about 59.5 mph on the speedometer. Compare them in the tire size calculator.

Which sizes work for common plus-size changes?

Table 1 lists nominal diameters for two starting sizes. Check that a size is sold in the tire you want before you decide.

The sidewall gets shorter with every inch of wheel. On the car example it falls from 112.8 mm to 78.8 mm, which leaves less rubber between the wheel and the road.

What else do you need to check besides diameter?

• Load index. The new tire must carry at least the load listed on the vehicle's tire placard. A lower-profile tire in the same width can have a lower load index, and some sizes are only sold as Extra Load (XL). See how to read a tire sidewall.
• Speed rating. Match or exceed the original speed rating.
• Rim width. Each tire has a rim width range approved by its maker. Check it for the new wheel.
• Clearance and offset. A wider tire or a wheel with a different offset can touch the suspension or the fender. Check the wheel maker's fitment data for your vehicle.

Frequently asked questions

Can I keep the same tire size on a bigger wheel?

No. The number after the R is the wheel diameter, so a tire for a 16-inch wheel only mounts on a 16-inch wheel. A bigger wheel needs a new tire size.

Do I have to make the tire wider when I go up a wheel size?

No. You can keep the width and lower the aspect ratio. A wider tire often gets the diameter closer because the aspect ratio only comes in steps of 5.

Can I go smaller instead (minus sizing)?

The same math works in reverse: a smaller wheel with a higher aspect ratio. Check that the smaller wheel clears the brakes. To see sizes near your current one, use the tire size alternatives tool.