215/75R15 vs 31x10.50R15: the usual TJ step
A Wrangler TJ's original tire against the first size most of them end up on.
31x10.50R15 is 3.30 in taller than 215/75R15 (11.93%) and 2.04 in wider. A speedometer calibrated for 215/75R15 would under-read by 11.93%.
Measured against each other
Overall diameter
27.70 in31.00 in+3.30 in (+11.93%)Section width
8.46 in / 215 mm10.50 in / 267 mm+2.04 in (+24.05%)Sidewall height
6.35 in8.00 in+1.65 inCircumference
87.0 in97.4 in+10.4 inRevolutions per mile
728.2650.6-77.6Ride height at the axle
——+1.65 inSpeedometer reading
——+11.93% under-read
| Dimension | 215/75R15 | 31x10.50R15 | Change |
|---|---|---|---|
| Overall diameter | 27.70 in | 31.00 in | +3.30 in (+11.93%) |
| Section width | 8.46 in / 215 mm | 10.50 in / 267 mm | +2.04 in (+24.05%) |
| Sidewall height | 6.35 in | 8.00 in | +1.65 in |
| Circumference | 87.0 in | 97.4 in | +10.4 in |
| Revolutions per mile | 728.2 | 650.6 | -77.6 |
| Ride height at the axle | — | — | +1.65 in |
| Speedometer reading | — | — | +11.93% under-read |
Calibrated for 215/75R15, running 31x10.50R15
- Dial reads 30
- 33.6 mph
- Dial reads 50
- 56.0 mph
- Dial reads 70
- 78.3 mph
- Odometer counts 100
- 111.9 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
3.30 in of diameter on an unchanged 15-inch wheel means all of it is sidewall, and the sidewall in question grows by more than a quarter.
Sidewall does all the work
A 6.35 in rubber column turns into a full 8.00 in one, and there is nothing else it could be: the rim is the same diameter, so every bit of the extra height is rubber, 1.65 in of it above the axle and 1.65 in below.
That is also the ride-height figure — 1.65 in at the axle centre — and it is the largest single contribution a tire change makes on this platform without any suspension work at all.
Just under twelve per cent
11.93% puts the pair firmly into the range where the arithmetic has to be carried around mentally rather than looked up. It is far enough out that the needle and a phone's GPS disagree visibly at any speed on any journey — which is usually how a TJ owner discovers the figure in the first place, rather than by working it out.
The width is the bigger proportional change
8.46 in to 10.50 in: 2.04 in, or 24.05% more casing. On a TJ that width arrives at the steering stop and the inner fender long before the height arrives anywhere, which is the usual pattern on short wheelbase four-wheel-drives and is worth separating from the diameter figure.
Where it sits on the ladder
This is the first rung. The second takes the same starting size to a 33 and reaches 19.15% — not double this, but close, and the consequences are considerably more than double because gearing and clearance both stop being optional.
The rung below, and why it is not the comparison
The flotation range does continue downward. 30x9.50R15 exists at 30.00 in, one inch shorter and one inch narrower than the 31. Against the same 215/75R15 that is 2.30 in and 8.32% rather than 3.30 in and 11.93%, on 1.04 in of extra section width rather than 2.04 in — roughly two thirds of the height step and half the width step. It is a real size and the arithmetic for it is on this site; it is simply the smaller of the two moves from the same starting point. Which of them is easier to buy in any given year is a question about stock, and nothing on this site can answer it.
The gearing conversion, for reference
Feed a 3.73:1 axle through the same ratio and the answer is 4.17:1; the nearest set anyone makes is 4.10:1. Start at 4.10:1 instead and the answer is 4.59:1, against an available 4.56:1. Those are conversions between numbers, and the gear ratio calculator will run whichever ratio your axles actually carry.