Tire Size Calculator

How the tire size calculator works

A tire size code is a set of measurements in shorthand. In a metric code such as 245/45R17, the first number is the section width in millimeters, the second is the aspect ratio (the sidewall height as a percentage of that width), the letter is the construction, R for radial, and the last number is the wheel diameter in inches. The calculator turns the code into the overall diameter, the height of each sidewall, the circumference and the number of turns per mile, then draws both tires to the same scale so you can see the difference instead of only reading it.

Used as a tire size comparison calculator, the first tab sets your current size against a new one and gives every difference in inches and in percent. The second tab works as a tire diameter calculator and tire height calculator for a single size, and the third finds the plus size that keeps your overall diameter on a bigger wheel. Think of the drawing as a tire size calculator visualizer: both tires stand on the same ground line, so a taller tire shows its extra height at the top.

What to enter and where to find it

• Tire size. Copy the code exactly as it is molded on the sidewall, including any P or LT in front. Federal tire rules require the size designation on the sidewall, and the original size is also printed on the tire placard and in the owner's manual. The load index and speed rating that follow it, such as 95W, don't change the size and can be left out.
• New tire size. The size you are considering. Systems can be mixed: an LT-metric size can be compared directly with a flotation size.
• Speedometer reading. Any speed you want translated. The calculator assumes the speedometer reads correctly on the current tire and shows the true speed on the new one.
• Plus sizing. The new wheel diameter in inches and the section width you want in millimeters, which is the first number of the new code.

P-metric, LT-metric, Euro-metric and flotation sizes

Size codes follow the nomenclature of the Tire and Rim Association in the US and the European Tyre and Rim Technical Organisation, the publications the federal tire standard points to for size designations. Every metric code shares the same geometry, so this tire calculator reads them all with one formula; only flotation sizes are different.

Reading each kind of code

• P-metric (P245/45R17): passenger-car tires to the US standard. The P names the class and doesn't change the dimensions.
• Euro-metric (245/45R17 or 245/45ZR17): no letter in front, same nominal dimensions as the P-metric size with the same numbers. A ZR or a speed rating doesn't change the size. A C after the wheel diameter marks a commercial van tire.
• LT-metric (LT265/75R16): light-truck tires, read exactly like the passenger codes.
• Flotation (33x12.50R15LT): wide light-truck sizes given in inches, overall diameter first, then section width, then wheel diameter. The diameter is printed in the code, and each sidewall is half of the diameter minus the wheel. The diameter in the code is a rounded nominal size; real tires sold as 33-inch often measure somewhat less, so check the maker’s figure.

Tire size formulas

Everything comes from the size code and two exact definitions: 25.4 millimeters in an inch and 63,360 inches in a mile. None of the formulas is empirical, so the only uncertainty is how closely a real tire matches its nominal size.

1. Sidewall height (in) = section width (mm) × aspect ratio ÷ 100 ÷ 25.4
2. Overall diameter (in) = wheel diameter + 2 × sidewall height
3. Flotation: overall diameter = first number; sidewall = (diameter − wheel diameter) ÷ 2
4. Circumference = π × overall diameter
5. Revolutions per mile = 63,360 ÷ circumference
6. Difference (%) = (new − current) ÷ current × 100
7. True speed = speedometer reading × new diameter ÷ current diameter, assuming the speedometer read correctly on the current tires

For the 245/45R17 in the first worked example, each sidewall is 4.34 in tall, so the tire stands 25.68 in, and the section width converts the same way to 9.65 in. These are nominal figures for a new tire on its measuring rim. A wheel narrower or wider than that rim changes the section width a little, and tread depth and construction move the real diameter by a few tenths of an inch, and flotation sizes often by more.

Revolutions per mile and the speedometer

Revolutions per mile is the number that links a tire to everything turning behind it. A tire with fewer revolutions per mile covers more ground per turn, so at the same road speed the axle, the driveshaft and the engine all turn slower: 785 against 756 in the first worked example is the same proportion as the change in diameter. As a revolutions per mile calculator, the tool works from the nominal diameter, dividing 20,168.1 by the diameter in inches.

Tire makers also publish a measured revolutions per mile figure, counted with the tire loaded on a vehicle at a steady speed. It usually comes out a few percent higher than the nominal figure, because a loaded tire rolls on a slightly smaller effective diameter than its unloaded size, and it differs between tire models of the same size. When the maker's number is available, it is the better one for gearing work in the gear ratio calculator or the rpm calculator. To use it there, enter 20,168 ÷ revolutions per mile as the tire diameter, in inches.

The speedometer counts turns as well, so its reading changes in proportion to the diameter. A taller tire makes it read low, meaning the car is moving faster than the needle shows, and a shorter tire makes it read high. The calculator reports that difference as data, for the reading you enter.

Plus sizing: rim size and wheel diameter

Plus sizing means fitting a bigger wheel with a lower-profile tire so the overall diameter, and with it the gearing and the speedometer, stays about the same. Used as a rim size calculator, the Plus sizing tab takes your current size, the new wheel diameter and the section width you want, and works out the exact aspect ratio that would keep the diameter.

Modern metric series come in steps of 5, so the exact figure almost never exists. The calculator lists the series on either side and highlights the one with the closest overall diameter. It is a wheel diameter calculator for the tire side only: whether the new wheel and the wider tire fit inside the wheel well is a job for the wheel fitment calculator and the wheel offset calculator.

Worked example

Three common questions, worked out with the same functions that run the calculator.

Common tire sizes and their dimensions

Nominal dimensions of popular passenger, performance and light-truck sizes, computed from their codes with the formulas above. Real tires of the same size differ by a few tenths of an inch between models, so use the maker's data when the fit is close.

Two sizes on the same wheel can differ a lot in height: a 70-series tire on a 17-inch wheel stands several inches taller than a 45-series tire on the same wheel. That is why the overall diameter, not the wheel size, is what the gearing and the speedometer respond to.

Common mistakes

• Reading the width as the height. In 275/40R17 the 275 is the section width in millimeters, not the diameter. Run the code through the calculator before using it in any gearing math.
• Treating the aspect ratio as a length. A 40-series sidewall is 40% of the section width, so a wider tire in the same series also has a taller sidewall and a bigger overall diameter.
• Comparing wheel sizes instead of overall diameters. A bigger wheel with a lower series can leave the diameter unchanged; the wheel size on its own says nothing about height.
• Mixing up the two flotation numbers. In 33x12.50R15 the 33 is the overall diameter and the 12.50 is the section width, both in inches.
• Expecting a real tire to measure its nominal size. Tires vary between models and shrink as the tread wears. For close comparisons use the maker's published diameter and revolutions per mile.
• Forgetting the odometer. A change in diameter shifts the odometer by the same percentage as the speedometer, so mileage logs and fuel economy figures change with it.