Tire Size Calculator

Compare two tire sizes

Type your current size and the size you are thinking of, exactly as it appears on the sidewall, such as 225/45R17 or LT265/70R17. Off-road sizes written in inches, like 33x12.50R15, work too. The slip shows both tires side by side to scale, with the numbers that decide whether the swap makes sense:

• Diameter difference. The single most important number. It changes your speedometer, odometer and effective gearing all at once.
• True speed. What you are really doing when the speedometer shows your chosen speed on the new tires.
• Sidewall and width. A shorter sidewall sharpens steering and rides firmer; a wider tread needs room in the wheel arch.
• Revs per mile. How many times the tire turns in a mile. Fewer turns means taller gearing and a lower cruising rpm.
• Wheel torque. How much the pushing force at the road changes in every gear. A bigger tire always means a little less.

Your car's original size is printed on the tire placard on the driver's door jamb, and in the owner's manual.

Tire size formulas

A metric tire size contains everything you need to work out the tire's dimensions. The section width is in millimeters, the aspect ratio is the sidewall height as a percentage of that width, and the rim size is in inches.

These are nominal sizes. Real tires of the same size differ slightly between brands and models, a wider or narrower rim changes the shape a little, and a tire carrying the car's weight squats, so the revs per mile a tire maker publishes is usually a little higher than the calculated figure.

Speedometer error table

Your speedometer counts wheel turns and assumes the original tire. Fit a tire that is 3% taller and every turn covers 3% more road, so you are going 3% faster than the needle says. The table shows your true speed for common size changes.

The odometer is off by the same percentage, and so is any fuel economy you work out from it. Cars approved under UN Regulation 39, used across Europe and in many other countries, must never show less than the true speed and may show up to 10% more plus 4 km/h, so a speedometer that read slightly high from the factory can still be on the safe side after a small increase in tire size. A large increase can make it read low, and many cars can be recalibrated by a dealer or with a programmer.

How a bigger or smaller tire changes 0-60 and gearing

A tire is the last gear in the drivetrain. Make it taller and the engine turns fewer times per mile, exactly as if you had fitted a numerically lower axle ratio. The effect works out with the same arithmetic as a gear change:

Worked example: a pickup going from 265/70R17 (31.6 in) to 285/75R17 (33.8 in) gets tires 7.0% taller. At an indicated 60 mph it is really doing 64.2 mph. Its 3.73 axle now behaves like a 3.48, with about 6.6% less wheel torque in every gear. Getting the original performance back needs a 3.99 ratio, so owners usually regear to the nearest common ratio, 4.10.

Car and Driver measured the weight effect in a 2010 test of a Volkswagen Golf on wheels from 15 to 19 inches, with tire sizes chosen to keep the overall diameter close. From the 15s to the 19s, 0-60 mph acceleration got about 4% slower and fuel economy about 10% worse, while the 18-inch setup gripped the skidpad at 0.89 g against 0.85 g on the 17s. The magazine's verdict: "If it's acceleration you're after, stick with the smaller, lighter wheels and tires."

To see what a new tire does to your speed in each gear and rpm at cruising speed, use the gear ratio calculator. For the effect of power and weight on your 0-60 time, try the 0-60 calculator.

How to read a tire size

Take a full size code such as P215/65R15 95H. Here is what each part means, following the Tire Industry Association's guide to the sidewall:

• PPassenger tire, often called P-metric. LT marks a light-truck tire. Sizes with no letter are Euro-metric, which can carry a different load than a P-metric tire of the same size.
• 215Section width in millimeters, from sidewall to sidewall.
• 65Aspect ratio: the sidewall is 65% as tall as the tire is wide.
• RRadial construction, the standard for car and light-truck tires. ZR is also radial, on tires built for high speeds.
• 15The rim diameter in inches that the tire fits.
• 95Load index: a code for the maximum load the tire can carry.
• HSpeed symbol: the maximum speed the tire is rated for. Look any letter up in our tire speed rating chart.

Off-road and truck tires are often sized in inches instead. A 33x12.50R15 is about 33 inches tall and 12.5 inches wide, on a 15-inch rim.

Plus-sizing rules

Plus-sizing means fitting a larger wheel with a lower-profile tire so the overall diameter stays the same. Tire Rack's rule of thumb is to add about 10 mm of width and take 5 to 10% off the aspect ratio for every inch of wheel diameter. Here are its example sizes, with the diameters this calculator works out:

• Keep the diameter close. Within about 3% keeps the speedometer error under 2 mph at 60 mph.
• Never go lower on load. Michelin says a tire's load rating "must always meet or exceed the value specified by the vehicle manufacturer."
• Check the speed rating. Match or beat the original speed symbol.
• Check clearance. A wider or taller tire must clear the suspension, brakes and wheel arch at full steering lock and full compression.

Tire sizes in inches

Nominal dimensions of common car, SUV and truck sizes, worked out with the formulas above. A popular "33-inch" truck tire is 285/75R17, not 265/70R17, which is only 31.6 inches tall.

Sources and method

Every dimension is geometry from the size code: sidewall = width × aspect ratio, diameter = rim + 2 × sidewall, and revs per mile = 63,360 ÷ (π × diameter). These are nominal sizes; real tires vary slightly by brand, rim width and load. Plus-size examples and rules come from Tire Rack and Michelin, and the speedometer limits from UN Regulation 39.

• Tire Industry Association: reading a tire sidewall
• Tire Rack: plus-size wheels and tires
• UN Regulation 39: speedometer accuracy
• Car and Driver: wheel size test, 15 to 19 inches

Updated October 2026. Built and checked by the 0-60calc Team. Found a mistake? Tell us and we will check it against the source.

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