Tire Size Calculator

What the numbers on the sidewall mean

A tire size is a three part code followed by a service description. Take 225/45R17 91H, one of the most common passenger sizes sold in the United States.

Two of these are in millimeters and two are in inches, which is where most of the confusion starts. The width and the sidewall are metric. The wheel is imperial. Any calculator that mixes them without converting will be wrong by a factor of 25.4, so this page converts the rim to millimeters first and then reports the finished height in both units.

The aspect ratio is not a measurement. It is a percentage of the width, which is why a 45 series tire on a 225 mm width has a different sidewall than a 45 series on a 275 mm width. That single fact explains why two tires labeled 45 can look completely different on the car.

Worked example: 225/45R17 in both units

Start with the sidewall. The aspect ratio is 45% of the section width, so the sidewall height is 225 x 0.45 = 101.25 mm, which is 101.25 / 25.4 = 3.99 inches.

Then the wheel. A 17 inch rim is 17 x 25.4 = 431.8 mm. The tire wraps around it with a sidewall top and bottom, so the overall diameter is 431.8 + 101.25 + 101.25 = 634.3 mm, or 634.3 / 25.4 = 24.97 inches.

Circumference is diameter times pi. With pi at 3.1416: 634.3 x 3.1416 = 1992.7 mm, or 78.45 inches. That number is the one the car feels, because it sets how far the vehicle travels per wheel revolution.

Revolutions per mile follow from it. A mile is 63,360 inches, so 63,360 / 78.45 = 807.6 revolutions per mile. A tire catalog will print a number close to that, usually within 1%, because real tires deflect under load and the published figure comes from a loaded rolling radius rather than a free one.

Now change one number. Move to 235/45R17 and the sidewall grows to 235 x 0.45 = 105.75 mm, the diameter to 643.3 mm, and the revs per mile drop to 796.3. That is 11 fewer turns per mile, a 1.42% taller tire, from a 10 mm wider tread.

Revs per mile, the speedometer and the odometer

A speedometer is calibrated to a specific number of revolutions per mile, chosen by the manufacturer for the size on the door jamb. Fit a taller tire and each revolution covers more ground, so at any indicated speed the car is moving faster than the gauge says. Fit a shorter tire and the opposite happens.

The error is the ratio of the two diameters. Going from 634.3 mm to 643.3 mm is 643.3 / 634.3 = 1.0142, or 1.42% taller. At an indicated 70 mph the real speed is 70 x 1.0142 = 71.0 mph, and over 12,000 odometer miles the car has covered 12,000 x 1.0142 = 12,170 miles. The odometer under-reports, which also means the vehicle looks like it has done less mileage than it has at resale time.

Most tire shops and plus sizing charts treat 3% as the ceiling for a diameter change. The reason is not the speedometer, which can be reprogrammed on many vehicles, but the systems that read wheel speed: anti-lock brakes, traction control, stability control and the tire pressure monitors all compare four wheels against each other and against an expected value. A large change on one axle can set warnings or change how the systems intervene.

This page sizes one tire at a time. If you are deciding between two sizes, the tire size comparison calculator puts both diameters side by side and reports the speedometer error directly, which is the faster path for a before and after decision.

Load index: what one tire can carry

The two digit number after the size is a code from a standard table, not pounds. The table below covers the range on most passenger cars and crossovers.

The four tire total is not the number to compare with curb weight. A vehicle's weight is not split evenly, the front axle usually carries more, and the legal limit is the gross axle weight rating on the certification label rather than the sum of the tires. For a 3,500 pound car, the average corner carries 3,500 / 4 = 875 pounds, well under the 1,356 pound rating of a 91, but the front axle could still be close to its own limit when the car is loaded.

One rule has no exceptions: never fit a tire with a lower load index than the placard calls for. A lower index can also be a lower sidewall strength even when the size matches, which matters on a loaded vehicle in summer heat. Higher is allowed, at the cost of a stiffer ride and sometimes more weight.

Speed rating letters, briefly

The letter after the load index is the speed rating, a tested maximum sustained speed at full load and the recommended inflation pressure.

The rating is a laboratory figure, not a driving suggestion. Its practical value is that a higher rated tire is built to shed heat better, so a V rated tire on a car that only calls for H will usually run cooler and steer more sharply, at the cost of tread life and price. Going the other way, fitting an S rated tire where the manufacturer specified V, is a downgrade in heat capacity even if nobody ever approaches 149 mph.

The National Highway Traffic Safety Administration notes that the load index and speed symbol are not required on every tire sold, and that the NHTSA tire guide is the place to start for the government's labeling and safety rules. Where both are marked, they appear together right after the size.

What a taller tire does to the car

Diameter changes three things at once, and only one of them is the speedometer.

Gearing. A taller tire is a longer final drive ratio. The engine turns fewer times per mile, which usually means slightly lower rpm at highway speed and slightly slower acceleration. A 1.42% change is invisible. A 5% change is noticeable in a small engine.

Clearance. Height grows by half the diameter change at the top of the tire, but the gap to the strut and the fender shrinks by the same amount, and a wider section also moves the inner sidewall closer to suspension parts. A 225/45R17 to 245/40R18 swap grows the diameter by 2.98% and adds 10 mm of width on each side, which is where rubbing starts on a lowered car.

Rotation and wear. A larger diameter means fewer revolutions per mile, so the tire lasts a slightly different number of miles for the same tread depth, and the spare, if it is a different size, becomes more mismatched than before.

A diameter within 1% of stock is effectively invisible to the car. Between 1% and 3% is where plus sizing lives, and above 3% is where the electronic systems and the bodywork start to complain. The 3% figure is a guideline used across the industry, not a federal limit.

The rest of the sidewall: DOT code, dates and grades

Three markings beyond the size matter when you buy.

The DOT code. Every tire sold in the United States carries a Tire Identification Number molded into one sidewall, and the last four digits are the week and year of manufacture. A code ending in 2325 was built in the 23rd week of 2025. The format is set in 49 CFR Part 574, and it exists so a tire can be traced in a recall.

Maximum load and pressure. The sidewall states the maximum load at a stated cold pressure. That is the tire's ceiling, not the vehicle's recommendation. The pressure to run is on the door jamb placard, and NHTSA is explicit that the placard wins.

UTQG grades. Treadwear, traction and temperature grades come from the Uniform Tire Quality Grading System. Treadwear is relative to a control tire graded 100, traction runs AA, A, B, C for wet stops, and temperature runs A, B, C for heat resistance. They are useful for comparing tires within a brand and weak for comparing across brands.

Also look for M+S or the three peak mountain snowflake if you drive in snow. M+S is a tread pattern claim, while the three peak symbol means the tire passed a specific snow traction test, and only the latter is accepted in states with winter tire requirements.

When to keep the size on the door jamb

The honest answer for most drivers is that the size printed on the placard is the best size, and the calculator is for understanding it rather than replacing it.

Stay with the stock size when the car is under warranty, when it has a staggered setup with different front and rear sizes, when the wheels are original, and when nothing about your driving has changed. NHTSA's buying guidance is to fit the same size as the original tires or another size the manufacturer recommends.

Change size deliberately when you are buying a winter set and want a narrower tread for snow, when you are plus sizing wheels and keeping the diameter inside 3%, or when the original size is discontinued and an equivalent is the only option. In each case, check three things the math cannot see: the rim width range the tire maker approves for that section width, brake caliper clearance on a smaller wheel, and the load index against the placard.

The calculator gives you sidewall height, diameter, circumference and revs per mile from the numbers on the sidewall. It does not know your rim width, your suspension, or whether the tire you are looking at is any good in rain. Those come from the tire's spec sheet and its test results.