235/75R15 vs 33x12.50R15 — four inches on a 15-inch wheel
The XJ Cherokee's larger factory size against the classic 33, and what a 15-inch wheel has to do to get there.
33x12.50R15 is 4.12 in taller than 235/75R15 (14.27%) and 3.25 in wider. A speedometer calibrated for 235/75R15 would under-read by 14.27%.
Measured against each other
Overall diameter
28.88 in33.00 in+4.12 in (+14.27%)Section width
9.25 in / 235 mm12.50 in / 318 mm+3.25 in (+35.11%)Sidewall height
6.94 in9.00 in+2.06 inCircumference
90.7 in103.7 in+12.9 inRevolutions per mile
698.4611.2-87.2Ride height at the axle
——+2.06 inSpeedometer reading
——+14.27% under-read
| Dimension | 235/75R15 | 33x12.50R15 | Change |
|---|---|---|---|
| Overall diameter | 28.88 in | 33.00 in | +4.12 in (+14.27%) |
| Section width | 9.25 in / 235 mm | 12.50 in / 318 mm | +3.25 in (+35.11%) |
| Sidewall height | 6.94 in | 9.00 in | +2.06 in |
| Circumference | 90.7 in | 103.7 in | +12.9 in |
| Revolutions per mile | 698.4 | 611.2 | -87.2 |
| Ride height at the axle | — | — | +2.06 in |
| Speedometer reading | — | — | +14.27% under-read |
Calibrated for 235/75R15, running 33x12.50R15
- Dial reads 30
- 34.3 mph
- Dial reads 50
- 57.1 mph
- Dial reads 70
- 80.0 mph
- Odometer counts 100
- 114.3 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
An XJ that left the factory on the taller of its two tire options is still 4.12 in short of a 33.
All of it is sidewall
A 6.94 in sidewall turns into a 9.00 in one. The wheel does not change, so every one of those 4.12 in of extra diameter is rubber — 2.06 in of it above the axle and 2.06 in below, which is also the ride-height figure.
A 9.00 in sidewall on a 15-inch wheel is very nearly as much rubber as the wheel has radius. That proportion is unique to 15-inch flotation sizes and is the reason they behaved the way they did off road, and the reason nobody would design one today.
What 14.27% does
Past about ten per cent the error stops being something to allow for and becomes something to convert. The same 14.27% is the size of the effective gearing change as well, so a 3.55:1 axle would have to become 4.06:1 to keep engine speed where it was, and 4.10:1 is the closest set manufactured. One number, two entirely different bills.
The width is a third again
9.25 in to 12.50 in: 3.25 in, or 35.11% more casing. On a unibody Jeep with a narrow track and no room behind the front wheel, that is the dimension that decides the outcome, and it decides it at full lock rather than while driving in a straight line.
Against the other factory option
XJs were also sold on 225/75R15, which is 0.59 in shorter still. Owners starting there face an even larger version of this jump or, more commonly, the intermediate step to a 31 first. Very few of these vehicles went straight to a 33 from stock in one move, and the arithmetic explains why.
The aspect ratios, worked out
The metric size states its proportion: 75. The flotation size states nothing, and comes out at a 72-series once you divide 9.00 in of sidewall by 12.50 in of width. So the two casings are close in shape and very far apart in scale — the 33 is essentially the smaller tire enlarged, rather than a different design of tire.
The tools, with your own numbers
The speedometer error calculator will produce the full speed table for this pairing, and it will take any other starting size if yours has already been changed once.