How much taller is 285/70R17 than 265/70R17?

Twenty millimetres of width bought an inch of height

Nothing in these two codes changes except the first number, and yet the diameter moves. That is the aspect ratio doing its work: both sizes are 70-series, so the sidewall is defined as 70% of whatever the section width happens to be. Widen the section from 265 mm to 285 mm and the sidewall grows by 70% of that 20 mm — 14 mm at the top of the tire and another 14 at the bottom. The sidewall goes from 7.30 in to 7.85 in, and 1.10 in of overall diameter appears without anyone touching the middle number.

Where the inch actually goes

Only half of it, 0.55 in, raises the vehicle: the axle sits at the wheel centre, so it moves up by the change in radius rather than the change in diameter. The other 0.55 in fills clearance above the tire. Width adds a further complication in a different plane — 0.79 in more section, call it four-tenths of an inch on each side of the tire, most of which matters at the inner edge where the suspension and frame are, not at the fender.

3.49% on the instruments

That is what the diameter change costs a speedometer calibrated for the smaller size. An indicated 70 mph becomes 72.4 mph, and 100 recorded miles become 103.5. Small enough that many people never notice it, large enough that it accumulates in a service record.

Why this particular step is so common

265/70R17 and 285/70R17 sit one width apart in the same 70-series family on the same wheel, which makes this the least disruptive way to gain an inch of tire on a 17-inch truck — no change of rim, no change of aspect ratio, no jump into flotation sizing. Drawn to scale the two look almost identical, which is the point: the step is deliberately small. If you want to know what re-gearing it would take, the 285/70R17 gearing figures work it through.