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Size comparisons

31x10.50R15 vs 33x12.50R15 — the sizes that defined an era

The classic 15-inch off-road step, compared directly rather than against whatever either replaced.

31x10.50R1531.00 in tall10.50 in wide33x12.50R1533.00 in tall12.50 in wide+2.00 in · +6.45%
Drawn to scale from the size codes. Solid outline is 31x10.50R15; dashed outline is 33x12.50R15. The bars below each tire are section width at the same scale.

33x12.50R15 is 2.00 in taller than 31x10.50R15 (6.45%) and 2.00 in wider. A speedometer calibrated for 31x10.50R15 would under-read by 6.45%.

Measured against each other

  • Overall diameter

    31.00 in33.00 in+2.00 in (+6.45%)

  • Section width

    10.50 in / 267 mm12.50 in / 318 mm+2.00 in (+19.05%)

  • Sidewall height

    8.00 in9.00 in+1.00 in

  • Circumference

    97.4 in103.7 in+6.3 in

  • Revolutions per mile

    650.6611.2-39.4

  • Ride height at the axle

    ——+1.00 in

  • Speedometer reading

    ——+6.45% under-read

Calibrated for 31x10.50R15, running 33x12.50R15

Dial reads 30
31.9 mph
Dial reads 50
53.2 mph
Dial reads 70
74.5 mph
Odometer counts 100
106.5 mi covered

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

For twenty years, almost every conversation about tires on a 15-inch wheel was a conversation about one of these two.

Two inches each way

Two inches of diameter, two inches of casing, and one inch of sidewall to account for half the height. The symmetry is the notation's doing — flotation codes name both dimensions, so a two-inch step in each is exactly what the names say and nothing has to be inferred.

Proportionally the two are not equal: 6.45% on the diameter against 19.05% on the width. The wider tire is much more different than the taller one.

What the swap does

An even inch at the axle — which is as tidy as these figures ever get, and a consequence of flotation notation naming whole inches rather than of anything about the tires themselves. Two inches of diameter, one inch of radius, exactly.

In gearing terms: 3.73:1 converts to 3.97:1 against an available 3.91:1, and 4.10:1 converts to 4.36:1 against an available 4.30:1. Both land close enough that plenty of vehicles took this step and kept the axles they had.

The shapes, once worked out

The profile is the one figure these codes leave out, and recovering it is worth the trouble here. The 31 comes out at a 76-series and the 33 at a 72 — so the taller tire is, proportionally, the squatter of the two. A reader assuming the bigger tire has the taller profile has it backwards, because the width grew faster than the height did.

Why the pairing endured

Because 15-inch wheels were what small four-wheel-drives came with, and because these two sizes are the only points on the scale that were ever stocked in real volume. The gap between them — 32x11.50R15 exists but was never common — meant owners went from one to the other rather than stepping through anything.

That is still the shape of the decision today for anyone running a vehicle old enough to be on 15s.

What the wheel underneath has to cope with

Going from 10.50 in of casing to 12.50 in is two inches of extra section on a wheel that has not grown in diameter, and the wheel's other dimension — its width — appears in neither code and is not recorded here. It matters because the further a casing is fitted from the reference rim its width was quoted on, the less that quoted width means. Mounted narrow, the casing bows upward instead of outward: it gives up some of the 12.50 in and takes back a fraction on the 33.00 in. That is one reason measured heights for this size vary as much as they do between owners, and it is settled by reading the width off the wheel rather than off a size chart.

The floor and the ceiling

Below: 30x9.50R15, one inch smaller in each dimension. Above: nothing widely made on a 15-inch wheel, which is why the 33 was where the ladder stopped and why larger tires meant larger wheels from then on.

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