Winter Tire Sizes: Same Size or Minus Sizing?

A winter tire size starts from the size your vehicle came with. Some drivers then choose a different size for a separate winter set. This guide explains how to check that choice.

Nothing here says any size is approved for any vehicle. Only your vehicle maker or a tire professional can say that.

Start with the placard

Federal rules require a tire information placard on the vehicle. It shows the tire size designation for the tires installed when the vehicle was first sold, and the maker's recommended cold inflation pressure. The rule puts it on the driver's side B-pillar, with a few fallback spots on the driver's door. USTMA says the specifications are usually on a label inside the driver's door, in the glove box or in the fuel door, and that the owner's manual should list them too.

Read the size from the placard, then compare it with the size on your tire sidewall. Our guide to reading a tire sidewall explains the numbers. To see the sizes listed for your make and model, use our tire sizes by vehicle pages, then confirm them against the placard.

The default: the same size

Honda Canada's owner's manual says to choose winter tires with "the size and load ranges that are the same as the original tires," and to mount them on all four wheels. Michelin says every tire on a vehicle should match in size, type, speed rating, load capacity and construction. Its exception is where the vehicle maker specified different front and rear sizes. Transport Canada also says to fit winter tires in sets of four.

For many vehicles the winter tire you want is made in the original size. If it is, this is the simplest route.

What minus sizing is

Minus sizing means going down a wheel diameter and choosing a tire with a taller sidewall, so the overall diameter of the tire stays close to the original. It is the reverse of plus sizing.

The usual reason is a separate winter set on its own wheels. The tire is often narrower as well. Here is the arithmetic, for illustration only. Sidewall height is the width times the aspect ratio. Overall diameter is the rim diameter plus two sidewalls.

• 225/65R17: sidewall about 146 mm, overall diameter about 724 mm (28.5 in).
• 225/70R16: sidewall about 158 mm, overall diameter about 721 mm. That is about 0.4 percent smaller.
• 215/70R16: sidewall about 151 mm, overall diameter about 707 mm. That is about 2.3 percent smaller.

These are examples of the method. They are not a statement that these sizes suit any vehicle. Run your own pair through the tire size calculator, which shows the diameter and speedometer difference. For background on the first size, see 225/65R17.

How close is close enough?

Keep the overall diameter close to the original. Michelin says all-wheel-drive and four-wheel-drive systems may need matching diameters in all positions. Our plus-sizing guide explains the 3 percent rule of thumb and what a diameter check does not cover. Your vehicle maker's guidance outranks any rule of thumb.

Narrower or not?

Many winter sets use a narrower tire, but we did not find a tire maker or vehicle maker source that recommends one. Honda Canada's manual says to use the original size and load. Michelin says tires on the same axle must be the same size.

Ask your vehicle maker or a tire professional before you change width, particularly if you drive an all-wheel-drive vehicle. Michelin says some makers specify a maximum circumference variation and that small differences can strain drivetrain components.

Load index

The load index must meet or exceed the original. Pirelli and Continental both say never to fit a tire with a lower load index than the vehicle maker specifies, and USTMA says a replacement should not have less load-carrying capacity than the original specification. Our guide to load index and speed rating explains the numbers.

Load index belongs to each individual tire, so a different size can carry a different load index. Compare the exact tire you are considering with the figure on your placard or in your manual, and ask your installer if you are not sure.

Speed rating

The load index rule is firm. The speed rating is more nuanced for winter tires.

Michelin UK says you should not fit a tire with a lower speed index than the original unless it is a winter tire. It says winter tires may not have the same speed rating, and that the driver must limit speed so as not to exceed the tire's maximum.

Two cautions:

• Michelin USA says not to mix speed ratings on a vehicle, because the vehicle's top speed is capped by the lowest-rated tire. Use the same rating on all four.
• Check your owner's manual first. The vehicle maker's tire requirements come first.

A speed rating is not a safe driving speed. In winter, conditions set the limit long before the tire does.

Other things to check

• Brake and wheel fit. A smaller wheel has to clear the brakes, and the wheel has to suit the vehicle. Ask the wheel maker or your installer to confirm fit for your exact vehicle before you buy.
• Pressure. The placard pressure is for the original size. Our plus-sizing guide covers what to ask an installer.
• TPMS. Ask your installer whether the wheels need their own sensors. See our TPMS guide.
• Chains. Chains and socks are sold by tire size and need clearance. See our winter car kit guide.

A short checklist

1. Write down the placard size, load index and speed rating.
2. Check whether the winter tire you want comes in that size.
3. If you are considering a smaller wheel, compare the overall diameter in the tire size calculator.
4. Compare load index and speed rating on the exact tire. Do not go below the original load index.
5. Check wheel fit and brake clearance with the wheel seller or installer.
6. Fit all four, and ask your installer whether the new wheels need their own TPMS sensors.

For timing and the swap itself, see when to put on winter tires. To decide whether you need them at all, read do I need winter tires.