What a tire size code actually tells you
A metric tire size gives you three numbers and none of them is the height of the tire. In 275/40R18, the 275 is the nominal section width in millimetres, measured at the widest point of the sidewalls on a specified rim; the 40 is the aspect ratio, which is sidewall height as a percentage of that width; the R means radial construction; and the 18 is the wheel diameter in inches. The overall diameter — the figure every gearing calculation needs — has to be assembled from all three.
That mixed-unit convention is not an accident. It is a compromise between the metric section width used by the European Tyre and Rim Technical Organisation and the Tire and Rim Association, and the inch rim diameters that the wheel industry never abandoned. So a single calculation has to convert millimetres to inches before it can add anything to the rim size.
American flotation sizes such as 33x12.50R15 do the opposite. They state overall diameter first, in inches, then section width in inches, then rim diameter. There is no aspect ratio at all, because the height is given directly. This is why light-truck tires are commonly discussed as “33s” and “35s” while passenger tires are discussed by their size code.
Four derived numbers matter downstream. Overall diameter feeds every gearing and speedometer calculation. Sidewall height decides ride quality, wheel protection and whether the tire clears the caliper. Circumference is distance per revolution. Revolutions per mile is the figure tire manufacturers publish and the one that speedometer and odometer calibration is written against.
The conversion, step by step
The sidewall comes first, because it is the only part that needs converting:
sidewall (in) = section width (mm) × aspect ÷ 100 ÷ 25.4
An inch is exactly 25.4 millimetres by international definition, so this conversion is exact rather than approximate. A 275/40 sidewall is 275 × 0.40 = 110 mm, which is 110 ÷ 25.4 = 4.3307 in.
A tire has a sidewall above the rim and another below it, so the overall diameter adds two of them to the rim:
D = wheel diameter + 2 × sidewall
For 275/40R18 that gives 18 + 8.6614 = 26.6614 in. Note how strongly the rim dominates: the two sidewalls contribute less than half the total on a low-profile fitment, which is why changing the aspect ratio by 10 points moves diameter far less on an 18-inch wheel than on a 15-inch one.
Circumference and revolutions per mile follow from geometry:
C = πD and revs/mile = 63,360 ÷ C
because there are 5,280 feet, hence 63,360 inches, in a mile. Every one of these figures is geometric: it assumes a perfectly circular, undeflected tire. In service the tire flattens where it meets the road, so it covers slightly less ground per revolution and turns slightly more times per mile than the geometry predicts. Manufacturers measure and publish the real figure, and where the two disagree, the published figure wins. Feed whichever diameter you trust into the rpm to mph calculator.
Worked example: 275/40R18 and 225/65R17 side by side
Start with 275/40R18.
- Sidewall in millimetres. 275 × 40 ÷ 100 = 110 mm.
- Sidewall in inches. 110 ÷ 25.4 = 4.3307 in.
- Overall diameter. 18 + 2 × 4.3307 = 26.6614 in, which is 26.6614 × 25.4 = 677.2 mm.
- Circumference. π × 26.6614 = 83.759 in.
- Revolutions per mile. 63,360 ÷ 83.759 = 756.5.
Now the same arithmetic on 225/65R17, a common crossover size.
- 225 × 0.65 = 146.25 mm of sidewall, which is 146.25 ÷ 25.4 = 5.7579 in.
- 17 + 2 × 5.7579 = 28.5158 in overall.
- π × 28.5158 = 89.585 in circumference.
- 63,360 ÷ 89.585 = 707.3 revolutions per mile.
The comparison is the interesting part. The 225 is the narrower tire and sits on the smaller wheel, yet it is 1.854 in taller overall, because 65% of 225 is a much bigger sidewall than 40% of 275. Fitting the 225/65R17 in place of the 275/40R18 would make the vehicle travel 28.5158 ÷ 26.6614 = 1.0696, or 6.96%, further per revolution — so a speedometer calibrated for the low-profile tire would read about 6.5% low, showing 65 mph when the vehicle is doing 69.5.
For a flotation size the first two steps disappear. A 33x12.50R15 has an overall diameter of 33.000 in by definition, so its sidewall is (33 − 15) ÷ 2 = 9.000 in, its circumference is π × 33 = 103.673 in, and it turns 63,360 ÷ 103.673 = 611.2 times per mile.
How to use the numbers you get
Compare diameters as a ratio, not a difference. Speedometer error, gearing change and effective final-drive change are all proportional. Going from 26.661 in to 28.516 in is a factor of 1.0696, so every one of those quantities moves by 6.96%. A half-inch difference on a 26-inch tire matters twice as much as the same half inch on a 35-inch tire.
Keep replacement diameters inside about 3% of the original unless you are prepared to deal with the consequences. That is the rough band inside which speedometer error stays small, ABS and stability-control wheel-speed models stay happy, and gearing feels unchanged. Beyond it you are into recalibration territory, and beyond about 15% you are usually into clearance and load-rating problems too.
Check the load index and speed rating separately. Diameter tells you nothing about whether the tire can carry the axle load. A taller tire is not automatically a stronger one, and light-truck sizes carry a very different load structure from passenger sizes at the same nominal dimensions.
Watch the section width against your wheel width. The section width in the code is measured on a specified measuring rim. Mounted on a narrower or wider wheel, the real section width and the real sidewall height both shift, which means the calculated diameter drifts by a small amount too. The size code cannot capture this; only a fitment chart can.
Prefer published revolutions per mile over the geometric figure whenever you are calibrating something. The geometric number is exact arithmetic on an idealised circle. The published number is measured on a loaded tire and is what your odometer will actually agree with.
Common tire sizes converted
| Size | Sidewall (in) | Diameter (in) | Circumference (in) | Revs per mile |
|---|---|---|---|---|
| 205/55R16 | 4.439 | 24.878 | 78.157 | 810.7 |
| 215/60R16 | 5.079 | 26.157 | 82.175 | 771.0 |
| 225/65R17 | 5.758 | 28.516 | 89.585 | 707.3 |
| 275/40R18 | 4.331 | 26.661 | 83.759 | 756.5 |
| 305/30R20 | 3.602 | 27.205 | 85.466 | 741.3 |
| 245/75R16 | 7.234 | 30.469 | 95.718 | 661.9 |
| 265/70R17 | 7.303 | 31.606 | 99.295 | 638.1 |
| 285/75R16 | 8.415 | 32.831 | 103.142 | 614.3 |
| 33x12.50R15 | 9.000 | 33.000 | 103.673 | 611.2 |
Every row is the formula evaluated at that size. Manufacturers' published figures for the same sizes differ slightly because they are measured on a loaded tire.
Assumptions and traps
- Section width is nominal, not measured. It applies on a specified measuring rim. Two tires marked 275 from different makers can measure several millimetres apart, and both are correct.
- Geometric revolutions per mile is not the published figure. A loaded tire deflects and turns more times per mile than an undeflected circle of the same diameter. Use the manufacturer's number for any calibration work.
- Flotation sizes are rounded. A tire sold as a 33 frequently measures nearer 32.5 in when new, and the marketing size is not a specification. Measure the tire or use the published diameter.
- Tread wear shrinks the tire. A full tread depth of 10/32 in disappearing to 2/32 in takes half an inch out of the diameter — about 1.5% on a 33-inch tire, which is half of the 3% substitution band.
- The R is construction, not a number. R means radial, B means belted bias, D means diagonal bias. It never enters the arithmetic, but a size with a letter you do not recognise may not be measured the same way.
- Do not mix a driven and non-driven tire diameter. Gearing and speedometer work uses the driven tire only; the axle gear ratio calculator assumes the same.
Where the diameter figure goes next
Overall diameter is an input to almost every other driveline calculation. Feed it to the speed from rpm calculator to convert engine speed to road speed in any gear, or to the axle gear ratio and final drive calculator to see the effect on cruise rpm across every gear and to solve for the axle ratio a tire upsize demands. Both of those pages assume the same geometric convention used here, so the numbers are consistent between them.
Diameter also sets the speedometer correction factor directly. If the vehicle was calibrated for diameter D0 and you fit diameter D1, true speed equals indicated speed multiplied by D1 ÷ D0, and the odometer under-records by the same proportion. That relationship holds for any vehicle whose speed signal comes from wheel or transmission rotation, which is effectively all of them.
On the performance side, tire diameter is one half of the effective gearing that decides where in the rev range a run finishes — the quarter mile ET calculator covers the power and weight half of that. And when a tire upsize is part of a wider modification budget, the vehicle depreciation calculator is a sobering check on what the vehicle is losing regardless.
The one thing no size-code calculation can give you is fitment. Whether a 285/75R16 clears the suspension at full lock and full compression depends on backspacing, wheel width, offset and body geometry, none of which appears in the code. The arithmetic on this page tells you what the tire is; only a physical measurement tells you whether it fits.