255/70R18 — what the step up from 255/65R18 does to gearing

A light-truck fitment one aspect step above the crossover size on the same wheel.

Metric notation · Light truck (LT-metric)

Overall diameter: 32.06 in; 814 mm
Section width: 10.04 in; 255 mm
Sidewall height: 7.03 in; 179 mm
Aspect ratio: 70%; sidewall as a share of width
Wheel diameter: 18 in; the hole in the middle
Circumference: 100.7 in; 2558 mm
Revolutions per mile: 629.2; geometric, not measured
Revolutions per km: 390.9; geometric, not measured
Typical load index: 108–129; 2205–4079 lb per tire

LT-metric tires use a stiffer casing at a much higher reference inflation — Load Range C, D or E, meaning 50, 65 or 80 psi. The index band reflects the pressure the casing is rated to hold, not a larger tire.

The load-index range shown is the band this CLASS of size is typically produced in — not a list of what is in stock for this exact size today. Load index and speed symbol are printed on the sidewall of the specific tire you are holding, and the required minimum is on the placard of the specific vehicle you are fitting. Read both.

Nearest real sizes in imperial notation, same wheel

33x12.50R1833.00 in tall+0.94 in
35x12.50R1835.00 in tall+2.94 in
37x12.50R1837.00 in tall+4.94 in

Compare any two sizes, including ones this site has no page for, in the comparison tool.

Platforms recorded with this size

Half-ton pickups and full-size SUVs in off-road and load-carrying specifications, on the 18-inch wheels those trims usually run.

Coming from 255/65R18

The size directly below it on the same wheel measures 31.05 in. The step is exactly 1.00 in and 3.23%, with the section width and the wheel both unchanged — only the aspect ratio moves, from 65 to 70, and the sidewall from 6.53 in to 7.03 in.

Revolutions per mile falls from 649.5 to 629.2. Since engine speed at a given road speed is proportional to that figure, the vehicle is now geared 3.23% longer than it was.

The ratio that would undo it

Restoring the original engine speed means multiplying the axle ratio by the ratio of the two revolution counts:

Original axle	Ratio that restores it	Nearest set manufactured
3.21:1	3.31:1	3.31:1
3.55:1	3.66:1	3.73:1
3.73:1	3.85:1	3.91:1

Two of those three land between available ring-and-pinion sets, which is the normal outcome: axle ratios are manufactured at intervals set by tooth counts, not by tire sizes. The gear ratio calculator runs the arithmetic for any starting ratio, and the axle ratio basics reference explains what the ratio is and is not.

This is arithmetic about ratios. Whether any of it applies to a particular vehicle depends on that vehicle.

Around thirty-two inches

255/65R19 measures 32.05 in and 255/75R17 measures 32.06 in — the same casing width on 19 and 17-inch wheels, both within 0.01 in of this size. 275/65R18 is 0.02 in taller at 32.07 in on the same wheel, with 0.79 in more section. Two flotation sizes marked 32 measure 32.00 in exactly.

Aggressive tread and the diameter it actually delivers

LT constructions dominate this size, and a large share of them carry all-terrain or mud-terrain patterns. Those routinely measure under their nominal diameter when new, because the moulded tread blocks and the shoulder profile differ from the smooth-crown geometry the code describes. The 32.06 in above is what the specification says; a tape across a new tire will often say slightly less, and the overall diameter and rolling radius reference explains where the discrepancy comes from.

Class: Light truck (LT-metric). Every figure above is computed from the size code, not stored — see how these numbers are calculated.