245/75R16 — the three-quarter-ton workhorse size

Dimensions

Overall diameter 30.47 in — 774 mm. Section width 9.65 in. Sidewall 7.23 in, which is 75% of 245 mm. Circumference 95.7 in and 661.9 revolutions per mile.

The 0.14-inch question

The single most common thing anyone wants to know about this size is how it compares with 265/70R16, and the answer is unhelpfully close: 30.61 in against 30.47 in. Fourteen hundredths of an inch, 0.45%, 0.07 in of ride height at the axle. On a truck that difference does not exist.

Where they genuinely differ is width. 265/70R16 measures 10.43 in at the section against this size's 9.65 in — 0.79 in wider — and it gets there with more sidewall as well, 7.30 in against 7.23 in, because 70% of a larger number is still a larger number. So the choice between the two is a choice about footprint and appearance, with the gearing consequences deliberately engineered out.

Both were factory fitments on the same generation of three-quarter-ton pickup, which is exactly why the pair keeps coming up.

The 75-series ladder on a 16-inch wheel

Four widths at 75-series on this wheel, spaced 20 mm apart:

Size	Overall diameter	Section width
225/75R16	29.29 in	8.86 in
245/75R16	30.47 in	9.65 in
265/75R16	31.65 in	10.43 in
285/75R16	32.83 in	11.22 in

Every rung is 1.18 in of diameter and 0.79 in of section width. That regularity is the reason 16-inch light-truck fitments are easy to reason about compared with the flotation sizes that overlap them — the metric ladder has an even tread and the imperial one does not.

Coming down one rung to 225/75R16 costs 1.18 in and 3.88%. Going up to 265/75R16 gains the same 1.18 in and the same 3.88%, because this size sits exactly between them.

Full-size spares

A great many of the trucks that carried this size carried it as the spare too, slung under the bed on a full-size steel wheel. That is worth knowing for a practical reason: a spare that has sat under a vehicle for fifteen years matches the road tires in every dimension on this page and matches them in nothing else. Age is the difference, and the size code has no field for it — the moulded DOT date code is where it is recorded.

Where a truck's spare is a different size from its road tires, the diameter mismatch is the relevant arithmetic, and the spare tire sizes reference covers it.

Seven and a quarter inches of sidewall

7.23 in is a lot of casing between the rim and the road, and it is what makes this a load-carrying size rather than a styling one. Deep casings also deflect further under load, which is why the 661.9 revolutions per mile computed here is a floor rather than an estimate: the revolutions per mile reference explains which figure belongs in which calculation.

What a working truck can actually carry on this size turns on inflation as much as on the casing, and that relationship is a table rather than a number — the same tire holds different weights at different pressures, which is why an LT-marked 245/75R16 and a passenger-rated one can share eight characters and be rated hundreds of pounds apart. The tire pressure and load tables reference sets the relationship out.

What the step up costs in engine speed

Move from this size to 265/75R16 and the rolling diameter rises 1.18 in. An indicated 70 mph becomes an actual 72.7 mph, and 100 indicated miles becomes 103.9. The vehicle is geared 3.88% taller than it was, whatever the axle in it.

The arithmetic that undoes that is the ratio of the two revolutions-per-mile figures. On a truck with a 3.73:1 axle, restoring the original engine speed at a given road speed would require 3.87:1. The nearest ring-and-pinion set actually manufactured is 3.91:1 — close, and not the same number. On a 3.42:1 axle the computed answer is 3.55:1, which is a set that exists off the shelf.

None of that is a recommendation about any particular vehicle; it is division. The gear ratio calculator runs the same numbers for any axle, and the axle ratio basics reference explains what the ratio is doing.

Below and above, in the other notation

31x10.50R16 measures 31.00 in — 0.53 in taller than this size and 0.85 in wider. 33x12.50R16 stands 33.00 in, which is 2.53 in and 8.31% up, with 2.85 in of extra section. Between this size and the 31 there is no widely-stocked intermediate step in either notation.