How to read a tire code
A modern tire sidewall carries a designation such as 275/60R20 116T, and it is not one unit system but two. The 275 is the section width in millimetres — the widest point of the inflated, unloaded tire, measured on a specified rim at a specified pressure, not the width of the tread. The 60 is the aspect ratio: the sidewall height expressed as a percentage of the section width, so this tire's sidewall is 60% of 275 mm, or 165 mm. The R means radial construction. The 20 is the rim diameter in inches. The trailing 116T is the load index and speed symbol, which this calculator does not convert but which you must match when changing size.
To get an overall diameter you convert the sidewall to inches and add it twice, once for the top of the wheel and once for the bottom, to the rim diameter. That gives the free diameter — what the tire measures inflated and off the ground. It is the figure every size-comparison chart is built on, and it is the right basis for comparing two sizes against each other.
The reason anyone needs it is that a vehicle's speedometer and odometer are calibrated to a rolling circumference. Change the tire diameter and both read wrong by the same proportion, in the same direction, permanently. A tire 4% larger than stock means the car travels 4% further per wheel revolution than the electronics assume, so the speedometer under-reads and the odometer under-counts.
The arithmetic, step by step
Four operations, in order:
- Sidewall height in millimetres. Multiply the section width by the aspect ratio and divide by 100. For 275/60: 275 × 0.60 = 165 mm.
- Sidewall height in inches. Divide by 25.4, which is exact. 165 ÷ 25.4 = 6.4961 in.
- Overall diameter. Double the sidewall and add the rim: (2 × 6.4961) + 20 = 32.9921 in.
- Circumference and revolutions. Circumference is π × diameter = 103.6478 in. A mile is 5,280 ft × 12 = 63,360 inches, so revolutions per mile is 63,360 ÷ 103.6478 = 611.30.
Comparing two sizes is one more division. The ratio of the new overall diameter to the reference one is the factor by which every rotational quantity shifts: the speedometer reading, the odometer count, and the effective final drive ratio the engine sees. Subtract one and multiply by 100 for the percentage.
One caveat about revolutions per mile. The figure above is geometric, computed from the free diameter. A loaded tire deflects, so its rolling radius is smaller than half its free diameter and it turns slightly more times per mile than the geometry predicts. Manufacturers publish measured revolutions-per-mile figures that are typically a little higher than the geometric value, and those are the numbers to use if you are calibrating an electronic speedometer precisely. For comparing two sizes, the geometric ratio is fine, because the deflection effect largely cancels between two tires of similar construction and load.
Worked example: fitting 275/60R20 in place of 265/70R17
A truck left the factory on 265/70R17 and you are considering a 20-inch wheel with a 275/60R20 tire.
- Stock tire. 265 × 0.70 = 185.5 mm sidewall; 185.5 ÷ 25.4 = 7.3031 in; diameter = (2 × 7.3031) + 17 = 31.6063 in.
- New tire. 275 × 0.60 = 165 mm; 165 ÷ 25.4 = 6.4961 in; diameter = (2 × 6.4961) + 20 = 32.9921 in.
- Difference. 32.9921 ÷ 31.6063 = 1.043847, so the new tire is 4.385% larger in diameter — 1.386 inches, or 0.693 inch more clearance needed at the top of the arch.
- Speedometer. At an indicated 60 mph you are actually doing 60 × 1.043847 = 62.63 mph. The speedometer reads low, so you would be 2.6 mph faster than you think.
- Odometer. The same 4.385% applies: 10,000 indicated miles is 10,438 actual miles. Over a lease with a mileage cap, that difference runs in your favour on paper and against you on wear.
- Revolutions per mile. Stock: 63,360 ÷ (π × 31.6063) = 638.11. New: 63,360 ÷ (π × 32.9921) = 611.30. The wheel turns 26.8 fewer times per mile, which is the same 4.2% in reciprocal form.
Note that the two percentages are not identical: diameter is 4.385% up while revolutions are 4.201% down, because they are reciprocals. 1 ÷ 1.043847 = 0.95799, a 4.201% reduction. Quoting one figure as if it were the other is a small but real error.
Deciding whether a size change is acceptable
Start with the diameter change. A swap within about 3% of the original diameter is the range most fitters treat as a straightforward substitution; beyond that, speedometer calibration, clearance at full lock and full suspension travel, and the effect on gearing all need checking individually. This calculator flags 3% for that reason, but the number is a working convention, not a legal limit — your local regulations and the vehicle manufacturer's own tolerance are what govern.
Then check what the code does not tell you. Load index and speed symbol must at least match the placard on the driver's door pillar; fitting a tire with a lower load index than the vehicle requires is unsafe regardless of the dimensions working out. Section width also governs whether the tire fits the rim you have: every tire size has a range of approved rim widths, published by ETRTO in Europe and the Tire and Rim Association in North America, and a tire mounted outside that range sits with the wrong sidewall profile.
Finally, remember that a bigger diameter changes the effective gearing. The engine turns fewer times per mile, which lowers cruising revs slightly and raises the torque the driveline must deliver for the same acceleration. On a vehicle with fixed gearing, that is felt as a loss of low-speed responsiveness — the axle gear ratio calculator works out the ratio change that restores the original effective gearing.
Common tire sizes and their geometric dimensions
| Size | Sidewall (in) | Overall diameter (in) | Revs per mile (geometric) |
|---|---|---|---|
| 195/65R15 | 4.990 | 24.980 | 807.4 |
| 205/55R16 | 4.439 | 24.878 | 810.7 |
| 225/45R17 | 3.986 | 24.972 | 807.6 |
| 225/65R17 | 5.758 | 28.516 | 707.2 |
| 245/75R16 | 7.234 | 30.469 | 661.9 |
| LT265/75R16 | 7.825 | 31.650 | 637.2 |
| 265/70R17 | 7.303 | 31.606 | 638.1 |
| 285/75R16 | 8.415 | 32.831 | 614.3 |
| 275/60R20 | 6.496 | 32.992 | 611.3 |
| 315/70R17 | 8.681 | 34.362 | 586.9 |
The first three rows show why rim size alone tells you nothing: a 15, a 16 and a 17 inch fitment here all land within a tenth of an inch of the same overall diameter, because the aspect ratio falls as the rim grows.
What the code does not tell you
- Section width is not tread width. The 275 is the widest point of the inflated sidewall on a specified measuring rim, and the contact patch is narrower. Two tires of the same nominal width from different makers can differ measurably in both.
- Actual diameter varies from the calculated one. The code gives a nominal geometry; moulded dimensions differ between manufacturers and change as the tread wears. A tire at its wear bars is smaller in diameter than a new one of the same size.
- Load index and speed symbol are not optional. Match or exceed the values on the vehicle placard. Dimensions that work out on paper do not make an under-rated tire safe.
- Rim width has an approved range. ETRTO and the Tire and Rim Association publish the permitted rim widths for each size. Mounting outside that range distorts the sidewall profile and changes the handling and the actual section width.
- Flotation sizes use a different scheme entirely. A code such as 33x12.50R15 gives overall diameter and section width in inches directly, with no aspect ratio. This calculator handles the metric scheme; for flotation sizes the diameter is simply the first number.
- Speedometer error compounds with the instrument's own bias. Regulations such as UNECE R39 require a speedometer to read at or above true speed, so vehicles typically leave the factory reading slightly high. Fitting a larger tire eats into that margin before it starts under-reading.
Related tire and drivetrain tools
A size change ripples through several other numbers. The most immediate is speed and distance: everything the speedometer and odometer report shifts by the diameter ratio, which is why the mph to km/h calculator is worth having alongside if you are cross-checking against a GPS reading in another unit. For gearing, the axle gear ratio calculator converts a diameter change into the differential ratio that would restore the original effective drive.
Pressure is the other half of tire behaviour, and it is temperature-sensitive: a tire inflated on a cold morning reads higher after an hour of driving, and the placard pressure is a cold specification. The tire pressure temperature change calculator quantifies that shift.
For the underlying unit conversion, the sidewall calculation is nothing more than millimetres to inches, and the mm to inches calculator covers that directly — useful when a wheel specification gives offset and bolt circle in millimetres while the tire gives rim diameter in inches, which is the same split this page navigates.