Loading Tyre Size Comparison…
Read it off the sidewall, for example 225/45R17.
Flotation sizes such as 31x10.50R15 are read too.
1.72%
225/45R17 rolls 634.3 mm; 235/40R18 rolls 645.2 mm, a difference of 10.9 mm.
This is geometry and only geometry. Whether a size can go on your car is decided by the vehicle manufacturer’s specification and by a tyre professional who can see the arch, the suspension and the load the vehicle is plated for — not by a percentage. Nothing here reads a load index or a speed symbol, and nothing here says a size will fit.
The rolling diameter used throughout is the unloaded geometric diameter, which is what the designation describes. A tyre under load flattens where it meets the road, so the distance it actually covers in one turn is a little less than π times this diameter, and it moves with pressure, tread depth and the weight on the axle. The difference is small and it is the same kind of small on both sizes, which is why the percentage between them survives it better than either figure does on its own.
A metric sidewall designation such as 225/45R17 is a shape written down. The 225 is the nominal section width in millimetres, measured across the widest part of the unloaded casing when it is mounted on its specified measuring rim. The 45 is the aspect ratio: the sidewall height expressed as a percentage of that width. The R marks radial construction, and the 17 is the diameter of the rim in inches, which is the one number on the sidewall still quoted imperially.
Everything else stamped alongside it — the load index, the speed symbol, the load range, an XL or reinforcement mark — describes what the casing is certified to do rather than what shape it is. Those markings are read past and discarded here, because none of them can be derived from geometry and none of them belong in an arithmetic answer.
Take 225/45R17. The sidewall is 225 × 45% = 101.25 mm. The rim is 17 × 25.4 = 431.8 mm across. There is one sidewall above the rim and one below it, so the rolling diameter is 431.8 + 2 × 101.25 = 634.3 mm, and the circumference is π times that, or 1,992.7 mm. Divide a mile into it and the tyre turns 807.6 times to cover one.
Now 235/40R18: a 94 mm sidewall on a 457.2 mm rim gives 645.2 mm and a circumference of 2,027.0 mm, which is 793.97 turns to the mile. The second is 10.9 mm larger, and 10.9 ÷ 634.3 is 1.72%. That single percentage is what every downstream consequence is a restatement of.
Half the diameter change is what happens at the axle: 5.45 mm of the extra sits under the wheel and lifts the hub, and the other half sits above it.
Off-road and light-truck fitments often use a flotation size instead, written 31x10.50R15. It is a different convention rather than a different kind of tyre: the first number is the overall diameter in inches, the second the section width in inches, the third the rim. There is no aspect ratio in it because the height is given directly.
So 31x10.50R15 stands 31 × 25.4 = 787.4 mm tall on a 381 mm rim, leaving a 203.2 mm sidewall on a 266.7 mm section — which works out at a 76.2% aspect ratio, derived rather than declared. Both formats reduce to a diameter in millimetres, which is why a flotation size and a metric one can be compared directly.
The speedometer and the odometer both count wheel revolutions, so both carry the percentage directly. Ride height moves by half the diameter change. Gearing changes in the same proportion, which is felt as slightly longer legs and slightly duller acceleration. The extra section width and the extra diameter both eat into whatever space exists at the arch, the strut and the liner.
What the number cannot reach is any question about whether the change is allowed. Fitment is set by the vehicle manufacturer for that specific vehicle, and confirmed by a tyre professional who can see the arch, the suspension travel and the weights the vehicle is plated for. This page performs arithmetic on two shapes; it makes no claim that either shape belongs on your car.
No threshold is offered here, and you should be suspicious of any site that offers one as a round number. The limit depends on the vehicle, its arches, its suspension travel, its load rating and the regulations where it is registered. The manufacturer publishes approved sizes for exactly this reason.
No. Section width is measured across the widest point of the casing, which is out at the sidewall bulge rather than at the road. The tread contact patch is narrower, and the actual section width shifts a little depending on how wide the rim it is mounted on happens to be.
Because a loaded tyre is not a circle. It flattens where it meets the road, so the loaded radius is less than half the geometric diameter, and it moves with inflation pressure, tread depth and the weight on that corner. Manufacturing tolerance shifts it too, which is why moulded revolution counts differ between brands in the same size.
Yes. Both designations describe the same physical shape using different conventions, and both reduce to an overall diameter in millimetres. Once each is expressed that way the comparison is the same subtraction as any other, and the resulting percentage means exactly what it would between two metric sizes.