Plus Sizing Tires: Load, Speed Rating, Pressure and the 3% Rule

What plus sizing means

Plus sizing is fitting a larger-diameter wheel with a lower-profile tire, so the wheel fills more of the wheel arch while the overall tire diameter stays close to the original. "Plus one" means a wheel one inch bigger, "plus two" two inches, and so on. People also change size to get a wider tire or a taller one for off-road clearance.

Continental's technical bulletin on plus sizing for passenger and light truck vehicles lists four aspects to consider in every case.

1. Load capacity

The replacement tire's load carrying capacity must be equal to or greater than the original equipment tire. The bulletin says to find the original capacity from the door placard or owner's manual, and if those aren't available, to consult the vehicle or tire manufacturer. Lower-profile tires in a given width can have lower load capacity, which is why some plus-size fitments need an extra load (XL) tire.

Check load index with our decoder.

2. Inflation pressure

Never use a pressure lower than the vehicle manufacturer's recommendation, even if the new tire has a higher load index, and keep any front-to-rear difference the manufacturer specifies. The bulletin explains that sometimes a pressure higher than the placard figure is needed for the new tire to carry the original tire's load, especially when replacing standard load tires with extra load tires. Never exceed the maximum pressure molded on the sidewall. If the pressure differs from the placard, the owner (and future owners) should be told.

3. Speed rating

The replacement tire's speed rating must be equal to or greater than the original equipment fitment.

4. Rolling circumference

Keep the original rolling circumference as close as possible. A different circumference can affect speedometer calibration, anti-lock brakes and stability control. The bulletin says that under most circumstances a change within plus or minus 3% needs no adjustments to those systems, and that if there is any question about a size, you should contact the vehicle manufacturer.

Our comparison tool works out the percentage for any two sizes and flags changes beyond 3%.

Worked example: plus one

A car on 205/55R16 moving to 17 inch wheels:

All three are within 3% by diameter, but that is only the circumference check. Each must also meet the load index and speed rating of the original tire and be approved for the new wheel's width, and the wheel itself must clear the brakes and suspension.

All-wheel and four-wheel drive

Continental recommends replacing all tires on AWD and 4WD vehicles with the same size unless the vehicle came with a staggered fitment (different front and rear sizes) from the factory.

Things size math can't check

• Clearance: wider tires and different wheel offsets can rub on suspension, fenders or liners at full lock or full compression.
• Wheel width: each tire size is approved for a range of rim widths in the tire maker's data.
• Brakes: larger brakes can limit how small an inside diameter a wheel can have; this matters mainly when going down in wheel size.
• TPMS sensors: new wheels may need sensors fitted or programmed.

NHTSA advises buying the size on the vehicle's placard or another size recommended by the vehicle manufacturer, and asking a tire dealer if you have doubts. Treat any other size as something to confirm with a professional on the vehicle.

Common questions

What is plus one sizing?

Moving to a wheel one inch larger in diameter and a lower-profile tire so the overall diameter stays about the same. For example, 205/55R16 to 225/45R17 changes the calculated diameter by only 0.4%.

Is the 3% rule a law?

No. It comes from tire maker guidance: Continental's bulletin says that under most circumstances a change within ±3% does not require adjustments to speedometer, ABS or stability systems.

Can I mix sizes on an all-wheel drive car?

Continental recommends the same size on all four tires of AWD and 4WD vehicles unless the vehicle came from the factory with different front and rear sizes.

Who should confirm a new size fits?

The vehicle manufacturer's approved sizes are on the door placard and in the owner's manual. For anything else, NHTSA suggests consulting a tire dealer, who can check clearance and load requirements on the vehicle.