35x12.50R20 — one upgrade, five different speedometer errors

The broadest upgrade list in the catalogue

Nine platforms name 35x12.50R20 among their common upgrades — more than any other size here — and they are listed with the computed figures. Five are half-tons, three are three-quarter-tons, and one is a full-size import. Not one of the nine ships it.

Five starting points

Those nine trucks do not share a factory 20-inch tire. They use five different ones, and the arithmetic of fitting this size varies accordingly.

Factory 20-inch size	Diameter	Step to 35x12.50R20	Change	Actual speed at an indicated 70 mph
275/55R20	31.91 in	+3.09 in	9.69%	76.8 mph
265/60R20	32.52 in	+2.48 in	7.63%	75.3 mph
275/60R20	32.99 in	+2.01 in	6.09%	74.3 mph
285/60R20	33.46 in	+1.54 in	4.59%	73.2 mph
275/65R20	34.07 in	+0.93 in	2.72%	71.9 mph

Top to bottom, that is a factor of three and a half in the size of the change. The thirteenth- generation F-150 on 275/55R20 gains 3.09 in; the Silverado 2500HD on LT275/65R20 gains 0.93 in and reads only 1.9 mph slow at 70.

Anyone quoting a single figure for what a 35 does to a speedometer has silently assumed one of those five rows. Which one matters more than the tire being fitted does.

The odometer version of the same table

Over a hundred indicated miles, the same five starting points give 109.7, 107.6, 106.1, 104.6 and 102.7 real miles. The heavy-duty truck at the bottom of the list will drift under three miles in a hundred; the half-ton at the top will drift almost ten.

That difference compounds against every distance-derived figure a vehicle reports, including service intervals and fuel-economy readouts, none of which are recalculated when a tire is changed.

The metric size that is taller than it

295/65R20 measures 35.10 in — 0.10 in more than this tire — on 11.61 in of section width, which is 0.89 in less. Fitting it in place of these makes an indicated 70 mph an actual 70.2 mph.

It is also listed as an upgrade on five platforms, three of which overlap with this size's list. Where the two appear together, the choice between them is almost entirely a width decision with a tenth of an inch of height attached.

325/60R20 goes the other way: 35.35 in, 0.35 in taller, and 0.30 in wider at 12.80 in of section width — the only catalogued 20-inch size wider than a 12.50 flotation tire without being a 38.

Below and above

Dropping to 33x12.50R20 is −2.00 in and 5.71% on identical section width, so an indicated 70 mph becomes an actual 66.0 mph. 285/65R20 at 34.59 in is only 0.41 in shorter and 1.28 in narrower.

Going up, 37x12.50R20 is +2.00 in and 5.71% again — the flotation ladder's two-inch step, in both directions from here, on unchanging width. 38x13.50R20 is +3.00 in and 8.57%, and adds an inch of section width with it.

Top rung for one group, middle rung for the other

Split the nine platforms by what else appears on their upgrade lists and they fall into two groups.

The half-tons — the F-150s, the Silverado 1500, both Rams — pair this size with 33x12.50R20. For them it is the larger of two flotation options and the end of the list.

The three heavy-duty trucks pair it with 37x12.50R20 and LT295/65R20 instead. For them it is the smallest of the three, and the 37 above it is a further 2.00 in and 5.71%.

Same tire, same wheel diameter, opposite ends of two different ladders, and the reason is what the trucks started on rather than anything about the tire.

The two-inch step is worth less every time you take it

Flotation sizes climb in whole inches, which makes the ladder look evenly spaced. As a percentage — and percentage is what the speedometer, the odometer and the gearing all respond to — it is not.

Coming up to this size from 33x12.50R20 is 2.00 in and 6.06%. Going on from it to 37x12.50R20 is the same 2.00 in and 5.71%. The step after that, to 38x13.50R20, is 1.00 in and 2.70%. Down at the bottom of the ladder, on a 15-inch wheel, 31x10.50R15 to 33x12.50R15 is 2.00 in and 6.45%.

Identical physical steps, shrinking consequences, because each one is divided by a larger starting diameter. It is the same reason the nine trucks above disagree so violently about what fitting this tire costs them: the inches are fixed and the percentage is not.