285/70R17 vs 33x12.50R17 — is the metric size really a 33?

The short answer

No, but nearly. 285/70R17 works out to 32.71 in of overall diameter. 33x12.50R17 is 33.00 in, because in imperial notation the first number is the diameter. The metric size is 0.29 in shorter — 0.89%.

If your question was about the speedometer, stop here: 0.89% is six tenths of a mile an hour at an indicated 70, and there is nothing further to say about it.

The difference nobody asks about, which is bigger

Section width. 285/70R17 is 11.22 in wide; 33x12.50R17 is 12.50 in. That is 1.28 in — more than four times the height difference, and the dimension that determines whether the tire looks aggressive from the front, whether it fills the arch, and whether it clears the inner fender at full lock.

People compare these two sizes on height because height is what the names imply. Then they fit one and notice the other thing.

Why the confusion exists at all

The two notations answer different questions. A metric code states the casing's shape — width and sidewall proportion — and leaves you to work out how tall it ends up. An imperial code states how tall it is and leaves the shape implicit; 33x12.50R17's derived aspect ratio is 64, which is not printed anywhere on the tire.

So "a 33" is a name for a height, and "285/70R17" is a name for a shape that happens to produce almost that height on a 17-inch wheel. They are not two ways of writing the same specification, and treating them as interchangeable is exactly the trap.

What follows from the numbers

Revolutions per mile: 616.6 for the metric size, 611.2 for the imperial one. Circumference 102.8 in against 103.7 in. Ride height at the axle differs by 0.15 in — you would not see it.

Coming from a common factory 265/70R17 at 31.61 in, the two look far more alike than they do against each other: +1.10 in to the 285, +1.39 in to the 33. Both are commonly described as "the 33-inch step" and both are, roughly.

What this comparison does not settle

Whether either fits. Clearance is a property of the vehicle and the wheel, not of the tire, and the 1.28 in of width is the dimension most likely to find a problem — inner fender, control arm, or steering stop — long before the 0.29 in of height finds anything. Load rating is a separate matter again: both codes are sold across a wide band of load indexes and with and without an LT prefix.

The arithmetic above is complete and exact. The fitment question is a different question, asked of a different object.

Which one people actually end up with

Availability decides this more often than arithmetic does. On a 17-inch wheel, aggressive all-terrain and mud-terrain patterns are far more likely to be offered in 33x12.50R17 than in 285/70R17, and highway and touring patterns are far more likely to be offered the other way round. So the choice is frequently made by which tread you want rather than by the two-tenths of an inch that separates the sizes.

That is worth naming because a great deal of argument about these two sizes is really argument about tread type wearing a geometry costume.

The wheel width question

11.22 in against 12.50 in. The 1.28 in between them is the largest number on this page and the one least visible in a table of diameters, because it is the dimension the wheel participates in. A wider casing is specified against a wider measuring rim than a narrower one, so the same wheel is not the same proposition for both of these — but which wheel, and what each tire is approved for, are two facts this site holds neither of. The approved widths are printed on the tire manufacturer's data sheet; yours is stamped on the wheel, the middle figure in a marking like 17x8 ET25.

Whatever those turn out to be, the direction is fixed. A casing on too narrow a rim is pinched at the bead and bulges at the top, so the printed width overstates it and the printed height understates it — and the wider casing has 1.28 in more to be pinched.

Reading measured diameters

Neither figure above is a measurement. Both are computed from the code, which is a specification of the casing rather than a description of a finished tire. Real examples of both sizes routinely measure a few tenths under, and marked flotation diameters are rounded hard: tires sold as 33s frequently measure closer to 32.7 in, which — pleasingly and confusingly — is roughly what the metric size computes to in the first place.

If a decision here turns on a quarter of an inch, the only reliable method is to measure the actual tires on the actual wheels, inflated, with the weight of the vehicle on them.