Tire Diameter Calculator

How to read a tire size like 225/45R17

Every number in a metric tire size is a measurement, and once you know what each one is you can work out the tire's height without leaving your chair.

• 225 is the section width in millimeters: how wide the tire is at its widest point, sidewall to sidewall, when mounted on the recommended rim.
• 45 is the aspect ratio. It is not a measurement; it is a percentage. The sidewall height is 45% of the section width.
• R means radial construction, which is standard on passenger cars and light trucks.
• 17 is the wheel (rim) diameter in inches. Note the mixed units, which is the main reason this calculation feels awkward by hand.

So a 225/45R17 is 225 mm wide with a sidewall that is 45% of 225 mm, mounted on a 17-inch wheel. The overall diameter is the wheel plus two sidewalls, top and bottom. That is the number that decides whether the tire fits, whether your speedometer is honest, and how many revolutions the tire makes in a mile. Load index and speed rating, which often appear after the size (for example 91V), are separate ratings and do not affect diameter.

The diameter formula, step by step

There are only three steps, but you have to keep millimeters and inches apart.

1. Sidewall in mm = width x aspect ratio / 100. For 225/45 that is 225 x 0.45 = 101.25 mm.
2. Rim in mm = rim inches x 25.4. For a 17-inch wheel: 17 x 25.4 = 431.8 mm.
3. Diameter in mm = rim + 2 x sidewall = 431.8 + 202.5 = 634.3 mm.

Convert to inches by dividing by 25.4: 634.3 / 25.4 = 24.97 inches. Circumference is diameter x pi, so 634.3 x 3.1416 = 1,992.7 mm, or 78.45 inches. Revolutions per mile is 63,360 inches divided by circumference: 63,360 / 78.45 = 807.6 revs per mile.

The calculator above runs all four numbers and also compares two sizes side by side, because in practice that is the question people have: not "how tall is this tire" but "how different is this tire from the one on the car now." The National Highway Traffic Safety Administration publishes the tire labeling rules behind the sidewall markings if you want to check a specific tire's official specifications.

Worked example: 225/45R17 against 235/45R17

This is the most common plus-size question: one step wider, same aspect ratio, same wheel.

A 1.42% diameter increase sounds trivial, and it is the kind of change tire shops make every day. But it has three consequences. Your speedometer and odometer now read about 1.4% low, so a 35 mph speed limit can be 35.5 mph on the dash. The car sits 4.5 mm higher, which is invisible. And the tire turns 11 fewer times per mile, which is a small gearing change in the direction of slightly lower engine rpm on the highway.

Where it gets risky is the width. A 235 tire is 10 mm wider, and at full steering lock that can mean contact with a strut, a fender liner or a brake line. Width clearance is a physical check, not an arithmetic one: measure the gap you have now and compare it with the increase. Most published tire size guides treat a 3% diameter change as the practical ceiling, and anything beyond that as a decision that needs a speedometer recalibration.

Circumference, revs per mile and your dashboard

Diameter is what fits; circumference is what your car's computers care about. Speedometer, odometer, trip computer, fuel economy display and, on many modern cars, the transmission shift points and ABS/ traction logic all assume a specific number of revolutions per mile.

Every size change moves that number in proportion. Going from 24.97 to 25.33 inches cuts revs per mile from 807.6 to 796.3, a 1.4% drop, and your dash readings drop with it. That is why the error is so easy to describe: speedometer error percentage = diameter change percentage. A 2% taller tire means every speed and distance on the dash reads 2% low; a 2% shorter tire means they read 2% high.

There is a real-world edge here. If your speedometer already reads slightly fast, as most do from the factory, a modestly taller tire can make it more accurate. If it reads exactly true, a taller tire makes you faster than the dash says, which is not the direction you want to be wrong in near a speed camera. Before trusting any of it, confirm the actual diameter of the tire on your car: published sizes are nominal, and real tires vary by a few tenths of an inch between brands and tread depths.

Plus sizing without breaking things

Plus sizing means going up one wheel diameter and down one aspect ratio step, keeping the overall diameter roughly the same. The classic swap is 225/45R17 to 245/40R18: the sidewall shrinks from 3.99 to 3.86 inches while the wheel grows from 17 to 18, and the diameter only moves from 24.97 to 25.72 inches. That is a 3.0% change, at the edge of what most guides recommend, and it buys a stiffer, sharper-feeling tire at the cost of ride comfort and pothole protection.

Three things to check before any change. First, diameter: stay inside about 3% of stock, or plan to have the speedometer recalibrated. Second, width clearance: at full lock, is there still daylight between the tire and the strut, and does the fender clear the tread at full compression? Third, load rating: a plus-size tire that carries less load than the placard in your door jamb is a downgrade no matter how it looks.

The load index matters more than most people realize. A taller, wider tire is often heavier, and if the new tire's load index is lower than the original, the tire is working closer to its limit. Check the placard on the driver's door jamb for the original size and pressure, and treat any change as a system: tire, wheel, pressure and alignment all move together.

What a diameter change does to wear, gearing and resale

A diameter change is a gearing change. A 3% taller tire makes every gear 3% taller, which means slightly lower rpm at highway speed and slightly less torque at the wheels in first gear. On a small four-cylinder, a 3% increase is noticeable when pulling away loaded or climbing a steep grade. On a truck it is usually lost in the noise.

The odometer is the part that costs money later. If the tire makes the odometer read 2% low, your recorded mileage falls 2% behind actual, which affects lease mileage limits, warranty terms and the resale value you can honestly claim. Over a 36,000-mile lease that is 720 miles of real wear that never appears on the odometer, and it goes the other way too: a shorter tire racks up recorded miles faster than the car is travelling.

Finally, mismatched diameters matter. On an all-wheel-drive or four-wheel-drive vehicle, tires of different diameters turn at different speeds, and the center differential or coupling has to absorb the difference continuously. A 1% diameter difference between front and rear is generally treated as the limit; beyond that you are heating a component that was not designed to slip. If one tire is replaced and the others are half worn, the diameter difference can already be in that range, which is why AWD cars are often sold in sets of four.