Plus Sizing: What Changes When You Go Up a Wheel Size

What plus sizing means

Plus sizing is when you move to a larger wheel diameter and fit a tire with a shorter sidewall, so the overall height of the wheel-and-tire stays close to the original. Trade press describes it as installing larger rims, wider tires and shorter sidewalls while aiming to keep the original overall diameter.

People do it for looks, or to fit a different tire. This guide explains how to check the numbers. It does not tell you what is right for your vehicle, because only the vehicle maker or a tire professional can do that.

Nothing here says any size combination is approved for any vehicle. Confirm your current size on the tire sidewall or the driver's door placard before you compare anything.

Why overall diameter matters

The speedometer, ABS, traction control and electronic stability control all work from wheel rotation. Trade press says that to avoid problems with them, replacement tires should have the same overall diameter, which means the same rolling circumference, as the originals.

A larger tire travels farther per revolution, and the speedometer then reads low. If the overall diameter shrinks, the reverse happens.

Four-wheel and all-wheel drive vehicles need extra care. One tire maker's technical bulletin says all four tires must be closely matched in diameter to avoid strain on the differentials and couplings.

How to work out overall diameter

You only need the three numbers in a size like 225/65R17.

1. Width is the first number, in millimeters (225).
2. Aspect ratio is the second, the sidewall height as a percent of the width (65).
3. Rim diameter is the last, in inches (17).

Sidewall height in millimeters is the width times the aspect ratio, divided by 100. Overall diameter is the rim diameter plus two sidewalls. Convert millimeters to inches by dividing by 25.4.

A worked example, for illustration only

Here is a comparison of 225/65R17 against 235/60R18, using that arithmetic. It is chosen to show a method, not to suggest it suits any vehicle.

• 225/65R17: sidewall about 146 mm (5.8 in), overall diameter about 724 mm (28.5 in).
• 235/60R18: sidewall about 141 mm (5.6 in), overall diameter about 739 mm (29.1 in).

The second size comes out about 2 percent taller. Because it is taller, the speedometer would read slightly low. At an indicated 60 mph, the true speed would be about 61 mph.

You can run this comparison in our tire size calculator and swap in your own sizes. For background on each size, see our pages for 225/65R17 and 235/60R18.

The 3 percent rule of thumb

Many people use a limit of about 3 percent difference in overall diameter. A 2025 trade report says the Tire Industry Association continues to publish a 3 percent limit as the last official guideline for tire replacements.

Other sources give a range. A 2007 article put it at 2 to 3 percent or less, depending on the tire or vehicle maker you ask.

Please treat this as a rule of thumb only. It does not mean that a size within 3 percent is acceptable, or that a size outside it is unacceptable for your vehicle. Your vehicle maker's guidance outranks it.

What the diameter check does not cover

Matching diameter is one test out of several. Here is what else needs to line up.

Load index

Load index must meet or exceed the original. Trade press says never to install a tire with a lower load rating than the original equipment tire, and always to choose one with an equal or higher rating. USTMA likewise says a replacement should not have a smaller load-carrying capacity than what was originally specified.

Load capacity depends on inflation pressure. One tire maker's bulletin says to compare the replacement's load capacity with the original at the pressure shown on the placard. A tire industry bulletin adds that the same size can have different load indexes.

So read the load index on the sidewall of the tire you are considering, and compare it with the one on your placard or in your owner's manual. If you are not sure, ask a tire professional.

Speed rating

The speed rating of the replacement should be equal to or higher than the original, according to trade press. One tire maker's bulletin says the same, if the vehicle's speed capability is to be maintained.

Wheel width

The tire has to suit the wheel. Trade press advises making sure the intended tire fits the intended rim width and following the tire maker's recommendation.

Clearance

A new wheel and tire can rub on the suspension, fenders or liners. A tire retailer suggests checking real-world clearance at full steering lock and full suspension compression before everyday driving.

Inflation pressure

The pressure on your placard is for the original tire size. Federal rules require the owner's manual to say that if your vehicle has tires of a different size than the one on the placard, you should determine the proper inflation pressure for those tires. Ask the installer or the tire maker how to do that.

TPMS and the speedometer

A tire retailer advises recalibrating the speedometer and TPMS after installing a different size. Trade press adds that the TPMS underinflation threshold can likely be reprogrammed to suit higher inflation pressures. Ask the installer what your vehicle needs.

Ride and handling

Modern Tire Dealer says a shorter sidewall makes the tire stiffer, so buyers who want a soft ride should expect a harsher one. It also says a lower sidewall flexes less in turns, which can give quicker steering response. The same article says the closer the rim is to the ground, the higher the chance of rim damage. Ask a tire professional how the change will feel on your vehicle.

Warranty

Bridgestone's 2019 US warranty manual requires the tire to be a size and speed rating equivalent to or greater than the vehicle maker's specification. Ask the tire seller how a non-original size affects tire and vehicle warranties.

A cautious process

1. Find your original size on the tire sidewall and door placard. Note the load index and speed rating.
2. Compare the sizes you are considering using the calculator.
3. Check the load index, speed rating and rim width of the tire you are considering.
4. Ask your vehicle maker or a tire professional whether the combination is acceptable, and check the clearance.
5. Ask about speedometer, TPMS and inflation pressure after the change.

If you only need to find a size that matches what you have now, use the tire finder. It starts from your vehicle or the size on your sidewall.