Most size comparisons report one number: the difference in overall diameter. Four things change when you change a size, they change independently, and the one that most often causes trouble is not diameter.
The sidewall arithmetic is on the tire size conversion page.
The four numbers
Comparing 265/70R17 with 285/70R17:
| 265/70R17 | 285/70R17 | Change | |
|---|---|---|---|
| Section width | 10.43″ | 11.22″ | +7.5 % |
| Sidewall height | 7.30″ | 7.85″ | +7.5 % |
| Overall diameter | 31.61″ | 32.71″ | +3.5 % |
| Circumference | 99.3″ | 102.8″ | +3.5 % |
Two observations that people miss:
Width changes more than diameter. Because the aspect ratio is a percentage, widening the tire raises it too — but the diameter change is diluted by the fixed wheel diameter underneath. Here the tire got 7.5 % wider and only 3.5 % taller.
Width is what rubs. Clearance problems at full steering lock are far more often a width and offset issue than a height issue. Comparing on diameter alone hides the change most likely to cause a problem.
What each change does
Overall diameter drives the speedometer and odometer, and the effective final drive ratio. A taller tire raises the gearing: slightly lower engine revs at a given speed, slightly slower acceleration, and more load on the transmission when towing.
Circumference is the same information as diameter and is what the vehicle's sensors actually see. Revolutions per mile is 63,360 ÷ circumference in inches — the number ABS and traction control are calibrated around.
Section width decides fender and suspension clearance, needs a rim within the tire's approved width range, and changes the contact patch shape.
Sidewall height decides ride compliance and how much the tire deflects under cornering load. A shorter sidewall is sharper and harsher; a taller one absorbs more and rolls more.
The 3 % rule
A widely used guideline: keep overall diameter within about 3 % of the original.
The reasoning is not arbitrary. Within roughly 3 %:
- Speedometer error stays within what most people never notice and most jurisdictions tolerate.
- ABS and stability control calibration remains close enough not to be confused.
- Effective gearing changes little enough not to affect drivability or transmission behaviour.
Above 3 %, all three start to matter, and above about 5 % recalibration is worth doing rather than optional.
Manufacturers stay within this band themselves — the optional tire sizes across a model range are usually within 3 % of each other, which is why they can share one speedometer calibration.
Staggered and mismatched fitments
Some vehicles fit different sizes front and rear. Two things to be careful about:
On an all-wheel-drive vehicle, front and rear rolling circumference must match closely. A mismatch means the axles are permanently trying to rotate at different speeds, which loads the centre differential or transfer case continuously. This is a real mechanical failure mode, and it is also why replacing only two tires on an AWD vehicle is discouraged — a worn tire and a new tire of the same size do not have the same rolling circumference.
On rear-wheel drive, a staggered fitment is normal and only needs each axle to match itself.
Comparing properly
For any size change, check in this order:
- Load index — must meet or exceed the vehicle placard. This is a safety requirement, and it fails silently.
- Speed rating — must meet the placard.
- Rim width — the new size must be approved for the wheel you have.
- Overall diameter — within 3 % ideally, and recalibrate beyond that.
- Section width — clearance at full lock and full suspension compression, not at rest.
- Offset, if you are changing wheels too, because it moves the whole assembly outboard or inboard.
Steps 1 and 2 are pass/fail. Steps 4 to 6 are judgement.
Measured versus calculated
All the arithmetic uses nominal sizes. Real tires differ by a few tenths of an inch, and section width in particular depends on the rim it is mounted on.
For a marginal clearance decision, use the manufacturer's measured specification for the specific tire model rather than the calculated nominal — the difference between two tires of the same nominal size from different makers can be larger than the difference you are trying to evaluate.
What this page does not cover
- Speedometer error arithmetic is on the main conversion page.
- Fitment approval. Nothing here says a size is safe or legal on your vehicle.
- Wheel offset and backspacing, which interact with width to decide clearance.
- Recalibration, which is vehicle-specific and sometimes needs a dealer tool.
Four numbers, not one. And width is the one that rubs.