Changing tire size: plus-sizing and alternate sizes

A metric tire size gives you everything you need to work out how tall the tire is. In 225/65R17, the tread is 225 mm wide, the sidewall is 65% as tall as that width, and the wheel is 17 inches. (For a full walk-through of the code, see how to read a tire size.)

Here is how some common alternatives compare with that 225/65R17. A change of a percent or two is typical when moving between sizes in the same family.

You will often hear that a new size must be within 3% of the original diameter. It is worth knowing what kind of rule that is. BFGoodrich states it plainly as a guideline: keep the overall diameter within about 3 percent, beyond which speedometer and odometer error become significant. Continental gives no number and says only to stay as close to the original rolling circumference as possible. Michelin gives no percentage either and points owners to the automaker for restrictions.

Trade press reports that several tire makers once published the 3% figure and few still do, but it lives on in the Tire Industry Association’s technician training, partly because it has been cited in lawsuits. So treat it as a sensible industry convention, not a law. You may also see figures like “+3% / −2%” or “within 0.5 inch on AWD” online. We couldn’t find either one in a regulation or a tire or vehicle maker’s published guidance.

Plus-sizing means fitting a larger wheel and a lower-profile tire so that the overall height stays about the same. Each extra inch of wheel is a “plus”: a 16-inch to 17-inch change is Plus One, 16 to 18 is Plus Two. Toyo’s rule of thumb for each inch of wheel is about 10 mm more width and 10 points less aspect ratio. A Plus Zero change keeps the wheel diameter and goes wider with a lower aspect ratio, often needing a wider wheel.

Michelin and BFGoodrich describe the overall diameter as unchanged, but in practice it moves a little. Michelin’s own Plus Zero example, P195/75R14 to P215/65R14, works out about 2% shorter. The table below follows a common 205/55R16 through its plus sizes, with the load index and approved wheel widths that Continental publishes for one of its tire lines.

Two lessons sit in that table. First, a size can match the diameter almost perfectly and still fail: the 215/40R18 is within half a percent but carries less load than the tire it would replace. Second, plus sizes need wider wheels. The original 205/55R16 is approved on wheels as narrow as 5.5 inches, but the 225/40R18 needs at least 7.5, so the new wheels have to suit the new tire’s range. Federal rules require factory rims to match one the tire maker lists for that tire, and Continental tells owners to use only approved size and rim-width combinations.

Plus-sizing has costs as well. BFGoodrich lists a firmer ride, more risk of pothole damage, more road noise and more unsprung weight. Going the other way, minus-sizing for a winter set, works on the same math: a 195/65R15 is within 0.1% of a 225/45R17. The limit is brake clearance, since a smaller wheel has to fit over the calipers.

Every source agrees on this one. The U.S. Tire Manufacturers Association says never to choose a replacement with less load-carrying capacity than the original at the placard pressure, and the Tire Industry Association tells installers never to fit a lower load index than the original. Toyo calls it the golden rule of tire replacement.

The load index alone isn’t a reliable comparison, because capacity depends on pressure and on the tire’s construction. Lower-profile plus sizes often need an XL (extra load) build to match, and XL tires only reach their extra capacity at higher pressure. If a new size needs a pressure different from the placard, the right number comes from load and inflation tables, and you should never run below the placard pressure. On SUVs, pickups and vans, a passenger tire’s rating is also divided by 1.10 under federal rules. Our LT vs P-metric guide walks through that math.

Replacements should match or exceed the original speed rating, and Toyo adds that you shouldn’t mix ratings on one vehicle. The accepted exception is dedicated winter tires: USTMA and Michelin both say a lower-rated winter tire is acceptable as long as the vehicle is driven no faster than the tire’s rating. A good shop will tell you when it fits a lower rating. Our speed rating chart lists what each letter means.

Your speedometer counts wheel turns and assumes the original tire’s circumference. A taller tire covers more ground per turn, so the speedometer reads low; a shorter tire makes it read high. The odometer is off by the same proportion.

The same wheel-speed signal feeds anti-lock brakes and stability control, which compare how fast each wheel turns. Michelin notes diameter changes can affect gearing, traction and stability control, braking and the speedometer, and BFGoodrich adds that some vehicles calculate transmission shift points from wheel speed. Larger changes need professional recalibration.

Recalibration has limits. A General Motors service bulletin says GM only supports calibrations for tire sizes it tested for that vehicle, that changing the tire size in the ABS module does not correct the speedometer, and that the work is billed to the customer rather than covered by warranty. GM even refuses a size its menu lists if that size only fits with a different suspension package.

Your tire pressure monitoring system can be affected too. The federal TPMS standard makes automakers warn in the owner’s manual that replacement or alternate tires and wheels can stop the system working properly.

Many vehicles come in more than one tire size, usually split by trim. The 2025 Honda Civic Sedan LX runs 215/55R16, while the Sport and Sport Touring Hybrid trims run 235/40R18. Those two sizes are within 0.4% of each other in diameter, so Honda has effectively done a Plus Two inside one model.

A size the automaker already fits to your model has been through its calibration and testing. GM’s bulletin spells out the logic: if you can build the same model year, body, engine, transmission, axle and suspension with a given tire, that tire has a supported calibration. USTMA and Michelin both point owners to the original size or approved alternatives for the same reason. Another trim’s size usually needs that trim’s wheels, and its load index must still meet your own placard. Your owner’s manual lists the approved sizes on many vehicles.

Fitting non-factory tires doesn’t automatically void your vehicle warranty. The Federal Trade Commission’s guidance is that a manufacturer must show an aftermarket part caused the damage before denying coverage for it. A badly chosen or badly installed part that does cause damage can still be grounds for denying that repair.

Tire shops follow their own rules too. Industry training tells installers never to fit less load capacity than the original and not to go below the placard speed rating, which is why a shop may refuse a size that passes the diameter check. Those refusals protect you as much as them.