275/60R20 vs 35x12.50R20 — two inches on a half-ton

Two inches of diameter is the threshold at which a tire change stops being a tire change. Below it, people fit bigger tires and carry on. At and above it, the driveline starts being part of the discussion.

What two inches costs at the instruments

6.09%. Revolutions per mile falls from 611.3 to 576.2, which is the same statement in the units the vehicle actually works in — and the units every reading above is derived from.

Six per cent is comfortably enough to notice on a GPS and comfortably enough to matter to a maintenance schedule over a year. It is not, by itself, enough to make anything mechanical unhappy.

Where the gearing question comes from

The effective final drive falls by the same 6.09%, because engine speed at a fixed road speed is proportional to how many times the wheel turns. On a modern ten-speed automatic that is partly absorbed by the transmission having somewhere else to go, which is why this change is tolerated on half-tons far more readily than the same percentage would have been twenty years ago.

The arithmetic for restoring the original engine speed:

• from 3.55:1, the exact answer is 3.77:1, and the nearest ratio sold is 3.73:1
• from 3.73:1, the exact answer is 3.96:1, and the nearest ratio sold is 3.91:1

Both are conversions, not proposals — the ratio that reproduces one number given another. Run yours in the gear ratio calculator.

The sidewall gains an inch, and it is still short

An inch, from 6.50 in to 7.50 in. Worth putting in perspective: a 33 on a 17-inch wheel carries 8.00 in of sidewall, so this 35 — two full inches taller overall — has half an inch less rubber between rim and road than a 33 does on a smaller wheel. The 20-inch wheel takes that back before the tire has started.

Anyone moving to 35s specifically for sidewall is buying the wrong wheel diameter to do it on.

Width, and where it goes

10.83 in to 12.50 in, 15.45% more casing. Where that 1.67 in ends up is not a property of the tire at all: it is split between inboard and outboard by the wheel's offset, and offset is neither in a size code nor in this site's data. What can be said in general is that inboard and outboard run out of room against different things — control arms, liners and steering components on one side, the fender lip on the other — and that on a steered axle the tightest moment is at full lock rather than while driving. Which of those a given truck meets first is measured on the truck.

Two different kinds of tire, not just two sizes

275/60R20 is overwhelmingly sold as a passenger or hard-metric construction: a lighter casing, a lower reference inflation pressure, and load ratings calculated on the passenger-tire scale. 35x12.50R20 is overwhelmingly sold as an LT, with a heavier casing, a much higher reference pressure and a load index derived from it.

That is a change of construction on top of a change of dimension, and it is the part of this swap that the geometry has nothing to say about. Two tires of the same nominal height can differ by hundreds of pounds of rated capacity per corner depending on which scale they were rated on.

The rung below

If two inches is more than the truck can absorb, 275/60R20 against 33x12.50R20 is the same starting size against a 33 on the same wheel — and the result there is essentially no height change at all, with all of the 1.67 in of width and none of the diameter. The two pages together bracket the decision cleanly.

Ride height, stated properly

An even inch at the axle, which is unusually easy to remember and unusually easy to misread. It is not clearance: the space overhead shrinks by the same inch at the same moment, and the three radii a tire has explains why the tape measure will not agree with it either once the truck's weight is on the tire.

What is not decided here

Load index, wheel rim width, and whether the assembly turns. An LT-marked 35x12.50R20 is a heavy tire on a heavy wheel, and the unsprung mass added at each corner is real but is not a dimension any size code carries. The placard in your door jamb states what the vehicle requires; the arithmetic above states what the two tires measure, and those are separate questions asked of separate objects.