30x9.50R15 vs 31x10.50R15 — the flotation ladder starts here
The smallest genuine step in the flotation range, and a useful floor for reading the whole scale.
31x10.50R15 is 1.00 in taller than 30x9.50R15 (3.33%) and 1.00 in wider. A speedometer calibrated for 30x9.50R15 would under-read by 3.33%.
Measured against each other
Overall diameter
30.00 in31.00 in+1.00 in (+3.33%)Section width
9.50 in / 241 mm10.50 in / 267 mm+1.00 in (+10.53%)Sidewall height
7.50 in8.00 in+0.50 inCircumference
94.2 in97.4 in+3.1 inRevolutions per mile
672.3650.6-21.7Ride height at the axle
——+0.50 inSpeedometer reading
——+3.33% under-read
| Dimension | 30x9.50R15 | 31x10.50R15 | Change |
|---|---|---|---|
| Overall diameter | 30.00 in | 31.00 in | +1.00 in (+3.33%) |
| Section width | 9.50 in / 241 mm | 10.50 in / 267 mm | +1.00 in (+10.53%) |
| Sidewall height | 7.50 in | 8.00 in | +0.50 in |
| Circumference | 94.2 in | 97.4 in | +3.1 in |
| Revolutions per mile | 672.3 | 650.6 | -21.7 |
| Ride height at the axle | — | — | +0.50 in |
| Speedometer reading | — | — | +3.33% under-read |
Calibrated for 30x9.50R15, running 31x10.50R15
- Dial reads 30
- 31.0 mph
- Dial reads 50
- 51.7 mph
- Dial reads 70
- 72.3 mph
- Odometer counts 100
- 103.3 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Both numbers move by exactly one inch. That symmetry is a property of the notation rather than of the tires: flotation codes state diameter and width directly, so a step of one in each is precisely what it says.
The step, in full
Both codes promise an inch and both deliver one: 30.00 in to 31.00 in of diameter, 9.50 in to 10.50 in of casing. What neither code mentions is the sidewall, which grows half an inch — and since that half inch is the entire height gain per side, the rest of the arithmetic follows from it — the instrument readings above included.
What "one inch" is worth here proportionally
3.33% of diameter and 10.53% of width. The same absolute inch is worth three times as much proportionally on the narrow dimension as on the tall one, which is why flotation steps always feel like larger width changes than height changes even though the codes move in step.
Why 30x9.50R15 barely exists any more
Because it was superseded from below as well as above. A 30-inch tire is smaller than the metric sizes most light trucks now come with, and the vehicles that ran it — small pickups and utilities on 15-inch wheels — have largely gone. It survives as a trailer and light-utility size rather than as a serious option.
The 31, by contrast, remains one of the most produced flotation sizes ever made.
The proportions, worked out
Flotation notation has no field for a profile figure, so it has to be recovered. Divide sidewall by width and the smaller tire is a 79-series while the larger is a 76 — so the smaller one is proportionally the taller-profile tire, because its casing narrowed faster than its sidewall did. That is a general property of the flotation scale at its lower end, and it reverses further up.
Its use as a reference point
The value of the smaller size now is mostly arithmetic: it is the floor of the flotation scale, and knowing that the whole range runs from 30 inches to about 40 makes the intermediate sizes easier to place. A 33 is three rungs up from here; a 37 is seven.
The next step from the 31
31x10.50R15 against 33x12.50R15 is two inches of diameter and two of width — twice this step in both dimensions, and the change that defined the 15-inch off-road world.