Plus Sizing Tires: How to Upsize Wheel Diameter Correctly

What is plus sizing?

Plus sizing refers to the practice of fitting a larger diameter wheel (rim) to a vehicle while using a tire with a shorter sidewall to keep the overall tire diameter roughly the same. The rim diameter is the nominal diameter of the bead seat, as defined in Federal Motor Vehicle Safety Standard 110. By moving to a larger rim and a lower aspect ratio tire, you change the look and sometimes the handling without drastically altering the speedometer calibration.

In practice, a typical plus-size step moves from a 16-inch wheel to a 17-inch wheel, or from 17-inch to 18-inch, and reduces the tire's aspect ratio accordingly. The exact new size depends on the original section width and the available tire options. Understanding how tire sizes are written is the first step; see how tire sizes work.

Maintaining the overall diameter

The overall diameter of a tire is the outer diameter of an inflated tire, as defined in FMVSS 139. It is the sum of the rim diameter and twice the sidewall height. When you increase the rim diameter, you must reduce the sidewall height (by choosing a lower aspect ratio) to keep the outer diameter approximately the same. The goal is to make the difference between the original and replacement candidate as small as possible.

To check a candidate tire, you can compare the calculated outer diameters by using the section width, aspect ratio, and rim diameter. The section width is defined as the linear distance between the exteriors of the sidewalls, excluding any labels or decorations. For a deeper explanation of these measurements, see aspect ratio.

• Use a tire size calculator or manual formula to compare outer diameters.
• Aim for a difference of less than about one percent if possible.
• Always consider the actual rim width when verifying fit.

Speedometer and odometer effects

Speedometers and odometers are calibrated based on the number of wheel revolutions per mile. If the overall diameter changes, that revolution count changes, causing the speedometer to read either too high or too low. A larger overall diameter makes the tire cover more ground per revolution, so the speedometer reads a lower speed than the vehicle's actual speed. Keeping the original outer diameter avoids this error.

If you do accept a small diameter change, be aware that the speedometer error will be proportional to the percentage change. You may want to recalculate your fuel economy and trip distances as well.

Ride comfort and handling

A lower sidewall reduces the tire's sidewall flex, which can improve steering response and cornering stability. However, it also transfers more road imperfections to the cabin, resulting in a firmer ride. The shorter sidewall also leaves less rubber to absorb impacts from potholes and curbs, increasing the risk of wheel damage.

If you prioritize ride comfort, you may prefer to stay with a modest size increase such as 17 inches. If you want sharper handling and can accept a stiffer ride, 18 or 19-inch wheels are common choices. The trade-off is a classic one: responsiveness versus comfort. You can browse best 17-inch tires and best 18-inch tires to compare options.

Clearance and fitment

Larger wheels and lower-profile tires can affect clearance in several ways. The wheel may contact the brake calipers, suspension arms, or the inner fender liner. The wheel's width, offset, and bolt pattern must also be compatible with the vehicle. FMVSS 110 requires that rims be listed by the tire manufacturer as suitable for use with the tire, and that the tire's load rating be adequate.

Before purchasing, verify that the new wheel has enough backspacing to clear the suspension and that the tire does not rub when the steering is at full lock or when the suspension is compressed. Check the rim width range for the tire size you are considering. You can learn more in rim width range and wheel offset.

• Check for brake caliper clearance with the new wheel design.
• Test full steering lock to ensure the tire does not rub the fender liner.
• Consider the vehicle's load rating and tire pressure requirements.

Load rating and inflation pressure

When you change tire sizes, you must ensure the new tires meet the load requirements of your vehicle. Federal regulations specify that the sum of the maximum load ratings of the tires on each axle must be at least the gross axle weight rating. A plus-size tire may have a different load index, so you need to verify that it is rated for the same or greater load as the original.

Also, the recommended cold inflation pressure may be different for a plus-size tire. Always use the vehicle manufacturer's recommended pressure if it is listed for the new tire size, or consult a tire professional. Under-inflation can affect handling and fuel economy. See tire load index and tire pressure basics.

Choosing your plus-size tires

Once you have decided on a target wheel size, start comparing tires in that diameter. The 17-inch and 18-inch guides cover the most common upgrades, while 19-inch tires, 20-inch tires, and 21-inch and up tires serve larger wheels. Always cross-reference the tire size with the vehicle's original equipment and the tire manufacturer's rim matching information. A tire that is not listed for your rim width may not perform safely. Take the time to calculate the overall diameter and validate clearances before installing.

What to pick for your use

Questions

What is plus sizing?

Plus sizing is the practice of increasing the wheel diameter while reducing the tire sidewall height to keep the overall tire diameter similar. It improves the look and often handling, but you must verify fitment and load ratings.

Will plus sizing affect my speedometer?

If you keep the overall diameter the same, the speedometer stays accurate. If the diameter changes, the speedometer will read differently because the number of wheel revolutions per mile changes.

How much bigger can I go?

There is no single answer. It depends on the vehicle's available clearance, brake size, and the availability of tires in the right load range. A one-inch increase is common, but two inches may be possible with careful fitment checks.

Does plus sizing make the ride harsher?

Generally yes, because a lower sidewall transmits more road imperfections. The trade-off is improved handling and steering response.

Do I need to change tire pressure for plus-size tires?

Possibly. The recommended inflation pressure is based on the tire's load rating and the vehicle's weight. Follow the vehicle manufacturer's recommendation if a new size is specified, or consult a tire professional.

Can I plus size without changing the rim width?

You can keep the same rim width and change the tire size, but the tire must fit the rim width. Check the rim width range for the tire. Often, a plus-size step also involves a wider rim for optimal fit.