Tire Size Calculator

Plus Sizing: Same Height on a Different Wheel

Sizes we carry within 2% of your current tire's height, on a wheel one size smaller or up to three sizes bigger. Each one opens the tires we carry in that size.

No size we carry is that close on another wheel size.

Similar Sizes We Carry

Sizes we carry for a 17 inch wheel within half an inch of the new tire's height.

No other size we carry is that close for this wheel.

Gearing and Engine RPM

Your axle ratio3.083.213.313.423.553.733.924.104.304.564.885.135.385.71

At 65 mph in a 1:1 gear, your engine turns about 2,578 rpm on the current tire and 2,492 rpm on the new one. To get your current RPM back with the new tire, you would need about a 3.86 ratio. The closest common ratio is 3.92.

Overdrive gears lower every number here, but the change between the two tires stays the same.

How to Read a Tire Size

Take 265/70R17. The 265 is the tire's width in millimeters, about 10.4 inches. The 70 is the aspect ratio: the sidewall is 70% as tall as the tire is wide, which is 185.5 mm or 7.3 inches. The R means radial construction, and 17 is the wheel diameter in inches.

Flotation sizes, common on lifted trucks, read the other way around. A 35x12.50R20 is 35 inches tall and 12.50 inches wide, and it mounts on a 20 inch wheel. Switch either tire to Flotation above, or type the size, to enter one.

Everything else on the sidewall

A full truck tire size looks like LT285/70R17 121/118S. Here is what each part means:

• LT is the tire type: P for passenger, LT for light truck, ST for special trailer and T for a temporary spare. No letter usually means a metric passenger tire.
• 285/70R17 is the size: width, aspect ratio, construction and wheel diameter. ZR in place of R marks a radial built for high speed. D means bias ply and B means belted.
• 121/118 is the load index. The first number is for a single tire and the second for each tire on a dual rear wheel truck. 121 is 3,197 pounds per tire.
• S is the speed rating, the top speed the tire is built for: S is 112 mph.
• LT tires also carry a load range letter such as E, which tells you the ply rating and the most air pressure the tire is built to carry.
• The DOT code ends in four digits for the week and year the tire was made: 2324 means the 23rd week of 2024.

Load index (pounds per tire)

How Tire Diameter Is Calculated

Overall diameter is the wheel plus two sidewalls. For 265/70R17, one sidewall is 265 x 0.70 = 185.5 mm. Two sidewalls are 371 mm, or 14.6 inches. Add the 17 inch wheel and the tire is about 31.6 inches tall. Circumference is the diameter times pi, and revolutions per mile is 63,360 inches divided by the circumference.

Bigger Tires and Your Speedometer

Your speedometer counts how fast the wheels turn. A taller tire covers more ground on every turn, so the vehicle moves faster than the speedometer shows. Going from 265/70R17 to 285/70R17 makes the tire 3.5% taller, so an indicated 60 mph is really about 62.1 mph, and every 10,000 miles on the odometer is about 10,349 real miles.

A common rule of thumb is to stay within 3% of the original diameter so the speedometer stays close. Bigger jumps are normal on lifted trucks, and a speedometer recalibration puts the reading back.

Popular Tire Sizes in Inches

Calculated from the size on the sidewall. Measured diameter varies a little by tire model and tread depth.

Flotation Tire Sizes We Carry, in Metric

Every flotation size we carry, with its height, width and the metric size it works out to. Each size opens the tires we carry in it.

Will the New Size Fit?

Tire size is only half of fitment. Wheel width, offset and the space in your wheel wells decide whether a bigger tire rubs. These tools check the rest.

Tire Size Questions