Guide
Nominal vs actual tire diameter
Tire size calculators (including this one) work out a nominal diameter from the size code. Real tires differ: in the data books we use, makers' published diameters are typically within about ±1.5% of nominal, and flotation sizes (33x12.50R20 and so on) usually measure 0.5–1.5% under their name. Published revolutions per mile run 2.5–5% above the calculated figure because makers measure under load.
Calculated vs published, examples
| Size | Calculated | Published | Difference | Tire |
|---|---|---|---|---|
| P215/75R15 | 27.7 in | 27.6 in | −0.3% | Michelin LTX M/S |
| P225/75R15 | 28.29 in | 28.3 in | ±0.0% | Michelin LTX M/S |
| LT235/75R15 | 28.88 in | 28.8 in | −0.3% | Michelin LTX M/S |
| 31x10.50R15 | 31 in | 30.8 in | −0.6% | Michelin LTX M/S |
| 30x9.50R15 | 30 in | 29.6 in | −1.3% | Michelin LTX M/S |
| LT245/75R16 | 30.47 in | 30.7 in | +0.8% | Michelin LTX M/S |
| P275/70R16 | 31.16 in | 31.3 in | +0.5% | Michelin LTX M/S |
| P245/65R17 | 29.54 in | 29.7 in | +0.5% | Michelin LTX M/S |
| LT265/70R17 | 31.61 in | 31.7 in | +0.3% | BFGoodrich KO2 |
| LT285/70R17 | 32.71 in | 32.8 in | +0.3% | BFGoodrich KO2 |
| LT315/70R17 | 34.36 in | 34.4 in | +0.1% | BFGoodrich KO2 |
| LT255/70R18 | 32.06 in | 32.1 in | +0.1% | BFGoodrich KO2 |
| 37x12.50R17 | 37 in | 36.5 in | −1.4% | BFGoodrich KO2 |
Why they differ
- Each tread pattern and construction measures differently, and makers allow tolerances (Michelin's sheet notes about ±3% of section height).
- Width depends on the rim: Michelin notes section width changes about 0.2 in per 0.5 in of rim width.
- Revolutions per mile are measured under load, where the tire's rolling radius is slightly smaller than its free radius.
Every size page here shows the makers' published diameters next to the calculation, e.g. 265/70R17 or 33x12.50R20.
More guides
How to read a tire size · The 3% rule · AWD and mismatched tires · Load index & speed rating · Revs per mile chart · Tire size chart
Important: These are calculated (nominal) dimensions, not a fitment recommendation. Real tires of the same size differ by brand and model (makers allow about ±3% of section height), and width changes with the rim it is mounted on. The math can't see wheel width and offset, clearance at full steering lock and suspension travel, load index, speed rating, or what your vehicle maker approves. Before changing size, check your door-jamb placard and owner's manual and ask a tire professional. A replacement tire must meet or exceed the load index and speed rating on the placard.