285/70R17 — the metric size everybody calls a 33

What it is

285/70R17 is a light-truck size in metric notation: a nominal section width of 285 mm, a sidewall 70% of that width tall, on a 17-inch wheel. Work it through and it stands 32.71 in — which is why it is sold, described and argued about as a 33, and why it is not one. A genuine 33x12.50R17 measures 33.00 in exactly, because in imperial notation the diameter is the number printed on the tire. The gap is 0.29 in, or 0.89%.

That difference is small enough to be irrelevant to a speedometer and large enough to matter at a fender lip, which is a fair summary of why the question never dies.

Where it came from

The size exists because 17-inch wheels became the default on half-ton and mid-size pickups while the aftermarket was still selling 33s on 15s and 16s. Manufacturers wanted the same rolling diameter on a larger wheel — the factory answer to a customer who wants the look of a 33 without the manufacturer having to certify a flotation size. It is a factory-adjacent size that the aftermarket then adopted wholesale, which is unusual: most sizes travel the other way.

What is genuinely different about it

It is narrower than the 33 it substitutes for, by more than it is shorter. 285/70R17 is 11.22 in of section width against the 33x12.50R17's 12.50 in — 1.28 in narrower. People fixate on the 0.29 in of height and then discover the tire looks visibly less aggressive from the front. Width is the dimension that changes the appearance; height is the dimension that changes the arithmetic.

Its aspect ratio is honest, and the imperial size's is hidden. At 70, the sidewall is 7.85 in. The 33x12.50R17's derived aspect ratio is 64, and nothing on that tire tells you so.

It sits almost exactly one 20 mm step above the most common factory size on the same wheel. 265/70R17 is 31.61 in; the step to 285/70R17 is +1.10 in and +3.49%, which is the single most frequently performed tire change on a North American pickup.

The numbers that follow from it

Revolutions per mile is 616.6, from a circumference of 102.8 in. Fitting it in place of a 265/70R17 makes a speedometer under-read by 3.49%: an indicated 70 mph is an actual 72.4 mph, and an indicated 100 miles is 103.5. Against a 265/65R17 — the other very common factory size on this wheel, at 30.56 in — the error is 7.02%, and the ride-height change at the axle is 1.07 in.

Coming from 285/70R17, the next real step up in imperial notation on the same wheel is 35x12.50R17 at 35.00 in: +2.29 in and +7.01%, with the ride height rising 1.15 in.

Reading it on the shelf

Because the size spans passenger-adjacent SUV fitments and genuine light-truck fitments, the same code appears with and without an LT prefix and across a wide band of load indexes. The prefix is not decoration: an LT-marked tire uses a stiffer casing at a much higher reference inflation, and its load index reflects the pressure the casing holds rather than any change in shape. Two tires marked 285/70R17 can differ by hundreds of pounds of capacity each.

Check the load index on the sidewall against the minimum on your vehicle's door-jamb placard. That comparison is arithmetic, and it is the one people skip.

What it replaces, and what replaces it

Coming from below, the two sizes it most often replaces are 265/70R17 at 31.61 in and 265/65R17 at 30.56 in. From the first, the step is modest. From the second it is 2.15 in — over seven per cent — and it is the point at which a speedometer error stops being trivia: an indicated 70 mph becomes an actual 74.9 mph.

Going up, the honest next step is 35x12.50R17. There is no widely-sold metric size between the two on a 17-inch wheel that meaningfully splits the difference; 295/70R17 and 305/70R17 exist and are far less commonly stocked. In practice the ladder on this wheel is 33-ish, then 35, and this size is the bottom rung of it.

What 11.22 in is actually a measurement of

It is the section width the code specifies, and it is quoted on one reference rim of one stated width. That width is not in the code, it varies between individual tires marked 285/70R17, and it is printed on the manufacturer's data sheet rather than derivable from anything here.

Fit the casing on a wheel narrower than its reference and it draws in: measuring under 11.22 in and standing slightly taller than the computed diameter. Fit it wider and it squats and spreads. The working rate is about 0.2 in of section width per half inch of rim, in either direction, which is enough to explain most of the disagreement between two owners comparing tape measurements and concluding that one of them has a different tire. The width of the wheel is stamped on it, usually on the back of a spoke.