255/75R17 vs 35x12.50R17 — where speedometer error matters

Below about three per cent, tire-size arithmetic is a curiosity. Above about eight, it is something you have to actually account for. This pairing sits at 9.17% and is therefore the point on the ladder where the conversation changes.

What 9.17% does

The reading worth adding to the panel above is the one it does not carry: 60 on the dial is 65.5 on the road.

Sixty is the number that catches people, because it is a speed drivers set deliberately rather than drift into — a limit, a cruise-control detent, a roadworks sign. Five and a half miles an hour on a figure somebody chose on purpose lands differently from the same 9.17% at a speed nobody was aiming at, even though it is the identical error.

Fuel economy inherits the error twice over — the distance is under-counted, and the fuel is not — so a trip computer reading after this change is not comparable with one from before it.

The two sizes are not the same shape

Both stand on the same 17-inch wheel and there the similarity stops.

• Section width: 10.04 in becomes 12.50 in. That is 2.46 in, or 24.51% more casing.
• Sidewall: 7.53 in becomes 9.00 in.
• Aspect ratio: 75 on the metric size, a derived 72 on the flotation size — almost the same proportion, achieved at a very different scale.

The last line is the interesting one. These two tires are close to geometrically similar: the flotation size is very nearly the metric size scaled up. That is unusual, and it is why the step looks so natural in photographs while being so consequential in arithmetic.

Two platforms, one starting point

255/75R17 turns up as a factory size on the Wrangler JL and on the Bronco, which is why this particular pairing gets asked about far more than its position on the ladder would suggest. The vehicles are different in almost every respect that matters to fitment — suspension design, track width, the shape of the arch — and identical in the only respect this page measures, which is the tire they started with.

That is the argument for one page per pairing rather than one per vehicle. The arithmetic does not know which badge is on the grille.

Ride height, and what it is not

Half of the diameter change lands underneath the wheel centre, so the body sits 1.47 in higher relative to the ground. That figure is quoted as though it were a lift and it is not one: every clearance measured from the body to the tire is unchanged, and every clearance measured from the tire to the arch has got worse by the same 1.47 in. A suspension lift and a tire change move different things, and the three radii a tire actually has covers why the tape measure will not agree with either figure once the vehicle's weight is on it.

The count behind every other figure

Each wheel turns 629.1 times per mile on the factory size and 576.2 times on the 35. Fifty-three fewer revolutions per mile is the primitive fact here; the speedometer error, the odometer drift, the change in engine speed and the change in effective final drive are all the same ratio wearing different units. Working from the revolutions figure rather than from the diameter makes that obvious, and it is the number the vehicle's own sensors are actually producing.

The gearing conversion

The conversion returns 4.07:1 for a Jeep starting at 3.73:1, where 4.10:1 is the set that exists, and 4.48:1 for one starting at 4.10:1, against an available 4.56:1. Those are arithmetic equivalences and not proposals — the gear ratio calculator will run them against whatever your vehicle actually has.

The smaller version of the same move

255/75R17 against 33x12.50R17 is the same starting size stepping only to a 33: 0.94 in and 2.94%, roughly a third of the change described here for the same 2.46 in of extra width. The width is identical between the two comparisons; only the height differs. If width is your constraint, the two pages describe the same problem.

What is not settled

Whether either clears. That depends on wheel backspacing, on what the suspension does at full compression, and on where the bump stops sit — none of which appears in a size code. The figures above are exact and they are exactly about two tires, not about a vehicle.