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Size comparisons

265/65R18 vs 275/70R18 — height without much extra casing

An 18-inch Silverado step where the profile does the work and the arch, not the steering stop, is the constraint.

265/65R1831.56 in tall10.43 in wide275/70R1833.16 in tall10.83 in wide+1.59 in · +5.05%
Drawn to scale from the size codes. Solid outline is 265/65R18; dashed outline is 275/70R18. The bars below each tire are section width at the same scale.

275/70R18 is 1.59 in taller than 265/65R18 (5.05%) and 0.39 in wider. A speedometer calibrated for 265/65R18 would under-read by 5.05%.

Measured against each other

  • Overall diameter

    31.56 in33.16 in+1.59 in (+5.05%)

  • Section width

    10.43 in / 265 mm10.83 in / 275 mm+0.39 in (+3.77%)

  • Sidewall height

    6.78 in7.58 in+0.80 in

  • Circumference

    99.2 in104.2 in+5.0 in

  • Revolutions per mile

    639.0608.3-30.7

  • Ride height at the axle

    ——+0.80 in

  • Speedometer reading

    ——+5.05% under-read

Calibrated for 265/65R18, running 275/70R18

Dial reads 30
31.5 mph
Dial reads 50
52.5 mph
Dial reads 70
73.5 mph
Odometer counts 100
105.1 mi covered

Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.

Four times as much height as width. This pairing adds an inch and a half of diameter from a single 10 mm width increment, because the profile moves five points at the same time.

Two changes, both pushing up

  • 265 mm to 275 mm: 0.39 in of extra casing
  • 65-series to 70-series: a taller sidewall as a fraction of a wider casing

Sidewall goes from 6.78 in to 7.58 in — 0.80 in, most of which is the profile step. Doubled, that is the 1.59 in of extra diameter, and because the wheel is unchanged the axle inherits the sidewall gain exactly.

What the truck sees

5.05% of error, which is one recorded mile in twenty going missing. At town speeds the gap sounds trivial because a small proportion of a small number is a small number — but the proportion is identical at every speed above that, which is the part people misjudge when they test a swap in a car park and conclude it is nothing.

Put a 3.73:1 axle through the ratio and 3.92:1 comes out. A 3.91:1 set exists, which makes this one of the tidier conversions anywhere in this comparison set — and almost nobody performs it for a five per cent change.

Where the constraint moves to

Upward. With only 0.39 in of extra width, the space beside the tire is essentially unchanged, and the whole of the demand lands on the arch and on what sits above the tire at full compression. That is the reverse of most upgrade paths on full-size trucks, where width is the binding dimension.

Because it reaches a genuine 33-inch class — 33.16 in, slightly over a nominal 33 — while remaining a metric size available in highway, all-terrain and rugged patterns, and while asking for only 0.39 in more casing than the size below it. A flotation 33 on the same wheel diameter would ask for 12.50 in, which is 1.67 in more. That is the size of the difference; what either casing needs from a wheel in width terms is on its own data sheet and is not something a size code, or this site, records.

Circumference, for the record

5.0 in more ground per revolution, 99.2 in against 104.2 in — and unusually for this site, the difference lands on a round figure rather than near one. That is the physical quantity behind the whole page; the diameter and the percentage are two ways of restating it.

The flotation comparison

If the destination is a flotation 33 instead, the relevant figure is that 275/70R18 is actually taller than 33x12.50R18 by 0.16 in — a result that surprises most people, since one of the two has the number 33 in its name.

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