Tire Size Calculator

How to read a tire size

The size printed on a tire's sidewall, such as P225/65R17, packs in everything the tire size calculator needs:

• P, LT, ST, or T (optional) is the intended use: passenger car, light truck, special trailer, or temporary spare. It does not change the dimensions.
• 225 is the section width in millimeters: the distance between the outside of the two sidewalls on an inflated tire, not counting raised lettering, decoration, or protective ribs.
• 65 is the aspect ratio: the sidewall height as a percentage of the width. A 65 means the sidewall is 65% of 225 mm, or 146.25 mm.
• R means radial construction. Nearly all modern car tires are radial.
• 17 is the wheel (rim) diameter in inches that the tire fits.

How tire diameter and other dimensions are calculated

Sidewall height = width × aspect ratio ÷ 100
Overall diameter = wheel diameter × 25.4 + 2 × sidewall height
Circumference = π × overall diameter
Revolutions per mile = 63,360 in ÷ circumference in inches

For a 225/65R17 tire, the sidewall is 146.25 mm (5.8 in), so the overall diameter is 431.8 + 2 × 146.25 = 724.3 mm, or 28.5 inches. Its circumference is 89.6 inches, and it turns about 707 times per mile. These are nominal sizes; a real tire's height can differ slightly by brand, tread depth, and inflation.

Tire size comparison and your speedometer

Your speedometer and odometer count wheel revolutions, so they are calibrated for the original tire size. A taller tire travels farther each turn, so the speedometer reads lower than your actual speed. A shorter tire makes it read higher. Your actual speed changes by the same percentage as the overall diameter, measured against the speed the speedometer shows: comparing 225/65R17 with 235/60R18, the second tire is 2.06% taller, so when the speedometer shows 60 mph you are going 2.06% faster, about 61.2 mph.

Finding tire sizes for a new wheel size

When you move to a larger or smaller wheel, the goal is usually to keep the overall diameter about the same, so the speedometer, ABS, and stability control keep working as designed. Turn on “Find tire sizes for a new wheel size” and the calculator lists every size within 80 mm of your current width (in 5 mm steps) and with an aspect ratio of 35 to 70 whose overall diameter is within 3% of your current tire. A bigger wheel needs a lower aspect ratio to stay the same height.

Frequently asked questions

How tall is a 225/65R17 tire?

About 28.5 inches (724 mm) tall and 8.9 inches (225 mm) wide, with a 5.8-inch sidewall.

How much bigger can I go on tire size?

A common rule of thumb is to stay within 3% of the original overall diameter. Beyond that, speedometer error grows, and a larger tire may rub the fender or suspension. Your vehicle's door-jamb placard and owner's manual list the approved sizes.

Does a wider tire change my speedometer?

Only if it changes the overall diameter. With the same aspect ratio, a wider tire also has a taller sidewall, which raises the diameter. Use the comparison mode to see the exact difference.

Can I put a different tire size on my wheel?

The tire must match the wheel diameter, and its width has to suit the wheel's rim width. The tire manufacturer publishes the approved rim widths for each size, so check that before buying.

Things to keep in mind

Results are based on the nominal sizes in the tire code. The alternative sizes match only in dimensions; they may not be made by any manufacturer, fit your vehicle, or carry the load and speed ratings it needs. Talk to a tire professional before changing sizes.