225/75R16 vs 31x10.50R15 — a generation apart, not a swap
Two decades of Jeep fitment in one pairing, and why the factory caught up by changing the wheel rather than the tire.
31x10.50R15 is 1.71 in taller than 225/75R16 (5.85%) and 1.64 in wider. A speedometer calibrated for 225/75R16 would under-read by 5.85%.
Measured against each other
Overall diameter
29.29 in31.00 in+1.71 in (+5.85%)Section width
8.86 in / 225 mm10.50 in / 267 mm+1.64 in (+18.53%)Sidewall height
6.64 in8.00 in+1.36 inCircumference
92.0 in97.4 in+5.4 inRevolutions per mile
688.6650.6-38.0Ride height at the axle
——+0.86 inSpeedometer reading
——+5.85% under-read
| Dimension | 225/75R16 | 31x10.50R15 | Change |
|---|---|---|---|
| Overall diameter | 29.29 in | 31.00 in | +1.71 in (+5.85%) |
| Section width | 8.86 in / 225 mm | 10.50 in / 267 mm | +1.64 in (+18.53%) |
| Sidewall height | 6.64 in | 8.00 in | +1.36 in |
| Circumference | 92.0 in | 97.4 in | +5.4 in |
| Revolutions per mile | 688.6 | 650.6 | -38.0 |
| Ride height at the axle | — | — | +0.86 in |
| Speedometer reading | — | — | +5.85% under-read |
Calibrated for 225/75R16, running 31x10.50R15
- Dial reads 30
- 31.8 mph
- Dial reads 50
- 52.9 mph
- Dial reads 70
- 74.1 mph
- Odometer counts 100
- 105.8 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
These two tires never go on the same vehicle. The comparison is worth making anyway, because it measures what two decades of factory fitment actually delivered.
What changed between the generations
The older size is a 15-inch flotation tire: 31.00 in tall, 10.50 in wide, with 8.00 in of sidewall. The newer one is a 16-inch metric tire: 29.29 in tall, 8.86 in wide, with 6.64 in of sidewall.
The factory did not chase the aftermarket's diameter. It moved the wheel up an inch and fitted a narrower, shorter tire — which is the opposite of what the aftermarket had spent twenty years doing.
The gap that remains
1.71 in of diameter, or 5.85%, and 1.64 in of section width — 18.53%. Ride height at the axle differs by 0.86 in rather than the full 1.71, because the two tires sit on wheels of different diameters and half of the height difference lands below the axle line.
The number that is not what it appears
Sidewall. The 15-inch tire carries 8.00 in of it against 6.64 in — 1.36 in more, which is most of the diameter difference and all of the difference in how the two ride over broken ground. A 16-inch wheel takes an inch out of the available sidewall before the tire designer starts, and the JK's smallest factory size never got it back.
Why anyone looks this pairing up
Because owners cross-shopping a JK against an older Jeep want to know what the factory tire is relative to the size the platform is famous for, and because both sizes turn up in the same conversations about what "stock" means. The answer is that the modern factory size is smaller in every dimension, on a bigger wheel.
The equivalent step today
If the JK's own upgrade path is the question rather than the history, the useful comparison is 225/75R16 against 265/75R16 — a 31-inch class tire that stays on the 16-inch wheel, reached with a much wider casing instead of a taller sidewall. That is how the same height gets built once the wheel has grown.