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Size comparisons

215/75R15 vs 31x10.50R15 — the step almost every TJ takes

A Wrangler TJ's original tire against the first size most of them end up on.

215/75R1527.70 in tall8.46 in wide31x10.50R1531.00 in tall10.50 in wide+3.30 in · +11.93%
Drawn to scale from the size codes. Solid outline is 215/75R15; dashed outline is 31x10.50R15. The bars below each tire are section width at the same scale.

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 in

  • Circumference

    87.0 in97.4 in+10.4 in

  • Revolutions per mile

    728.2650.6-77.6

  • 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.

Nothing worth having sits below it

The flotation range does continue downward. 30x9.50R15 exists at 30.00 in, one inch shorter and one inch narrower, but it was never widely stocked and it is a smaller step than most owners are looking for. In practice the 31 is the bottom rung of the useful ladder on a 15-inch wheel, which is why this comparison exists and a gentler one does not.

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.

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