275/70R18 vs 33x12.50R18 — the metric size is the taller one
An 18-inch pairing that inverts the usual expectation, and a reminder that a nominal diameter is a name rather than a maximum.
33x12.50R18 is 0.16 in shorter than 275/70R18 (0.47%) and 1.67 in wider. A speedometer calibrated for 275/70R18 would over-read by 0.47%.
Measured against each other
Overall diameter
33.16 in33.00 in-0.16 in (-0.47%)Section width
10.83 in / 275 mm12.50 in / 318 mm+1.67 in (+15.45%)Sidewall height
7.58 in7.50 in-0.08 inCircumference
104.2 in103.7 in-0.5 inRevolutions per mile
608.3611.2+2.9Ride height at the axle
——-0.08 inSpeedometer reading
——-0.47% over-read
| Dimension | 275/70R18 | 33x12.50R18 | Change |
|---|---|---|---|
| Overall diameter | 33.16 in | 33.00 in | -0.16 in (-0.47%) |
| Section width | 10.83 in / 275 mm | 12.50 in / 318 mm | +1.67 in (+15.45%) |
| Sidewall height | 7.58 in | 7.50 in | -0.08 in |
| Circumference | 104.2 in | 103.7 in | -0.5 in |
| Revolutions per mile | 608.3 | 611.2 | +2.9 |
| Ride height at the axle | — | — | -0.08 in |
| Speedometer reading | — | — | -0.47% over-read |
Calibrated for 275/70R18, running 33x12.50R18
- Dial reads 30
- 29.9 mph
- Dial reads 50
- 49.8 mph
- Dial reads 70
- 69.7 mph
- Odometer counts 100
- 99.5 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Everyone expects the flotation size to be the taller of the two. Here it is not, by 0.16 in.
The inversion
275/70R18 computes to 33.16 in with a 7.58 in sidewall. 33x12.50R18 is 33.00 in by definition, with 7.50 in of sidewall. The metric size wins on both counts, narrowly.
The reason is simply that 70% of 275 mm is a fraction more than the (33 − 18) ÷ 2 that the flotation code specifies. There is no principle involved — the metric size happens to land a sixth of an inch above the round number the imperial size is named for.
What it means in practice
Nothing to the instruments. 0.47% of error, with the speedometer reading marginally high on the flotation tire — an error a set of tires will accumulate on its own between new and worn.
What it means conversationally is more useful: anyone who assumes a size must be shorter because it is metric has assumed wrong here, and would fit a "33" believing it to be an upgrade over a tire that is already taller.
The width is where they actually differ
10.83 in against 12.50 in — 1.67 in, or 15.45%. As is usual for metric-to-flotation pairings, the width gap is the substantial one and it is the dimension nobody checks first.
The derived aspect ratios diverge accordingly: the metric size states 70, and the flotation size works out to 60. Same height, very different proportions.
Why the 18-inch pairing looks like this
Because 18 is the wheel size at which metric truck sizes and flotation sizes overlap most closely. On 15s and 16s the flotation tires are much taller; on 20s they lose their sidewall entirely. At 18 inches the two notations meet, and 275/70R18 and 33x12.50R18 are the clearest illustration of it.
Availability decides it
Since the two are the same height to within a sixth of an inch, the choice is made on tread and on price. 33x12.50R18 is offered in a long list of aggressive patterns; 275/70R18 skews toward all-terrain and highway compounds and is the more common original fitment. Neither of those facts appears in a dimension table, and between them they account for nearly every decision made on this pairing.
What to compare next
275/70R18 against 35x12.50R20 is the step that actually changes something, and 265/65R18 against 275/70R18 is how most owners arrive at the metric size in the first place.