255/70R16 vs 33x12.50R15 — only works as arithmetic
A Bronco's base tire against a classic 15-inch 33 — a pairing nobody can actually perform, and a useful measurement anyway.
33x12.50R15 is 2.94 in taller than 255/70R16 (9.80%) and 2.46 in wider. A speedometer calibrated for 255/70R16 would under-read by 9.80%.
Measured against each other
Overall diameter
30.06 in33.00 in+2.94 in (+9.80%)Section width
10.04 in / 255 mm12.50 in / 318 mm+2.46 in (+24.51%)Sidewall height
7.03 in9.00 in+1.97 inCircumference
94.4 in103.7 in+9.3 inRevolutions per mile
671.0611.2-59.9Ride height at the axle
——+1.47 inSpeedometer reading
——+9.80% under-read
| Dimension | 255/70R16 | 33x12.50R15 | Change |
|---|---|---|---|
| Overall diameter | 30.06 in | 33.00 in | +2.94 in (+9.80%) |
| Section width | 10.04 in / 255 mm | 12.50 in / 318 mm | +2.46 in (+24.51%) |
| Sidewall height | 7.03 in | 9.00 in | +1.97 in |
| Circumference | 94.4 in | 103.7 in | +9.3 in |
| Revolutions per mile | 671.0 | 611.2 | -59.9 |
| Ride height at the axle | — | — | +1.47 in |
| Speedometer reading | — | — | +9.80% under-read |
Calibrated for 255/70R16, running 33x12.50R15
- Dial reads 30
- 32.9 mph
- Dial reads 50
- 54.9 mph
- Dial reads 70
- 76.9 mph
- Odometer counts 100
- 109.8 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
These two tires cannot be swapped. One needs a 16-inch wheel and the other a 15, and no modern Bronco is going to accept a 15-inch wheel over its brakes. What the comparison does is measure the gap between where the factory starts and where the old off-road standard sat.
The gap
2.94 in of diameter — 9.80% — and 2.46 in of section width, which is 24.51% more casing. Sidewall goes from 7.03 in to 9.00 in, nearly two inches more, and that is despite the wheel getting an inch smaller rather than larger.
That last point is the interesting one. The 15-inch flotation tire is taller overall while sitting on a smaller wheel, which means the entire 2.94 in plus the missing inch of wheel — very nearly four inches — has to come out of sidewall.
Why the pairing gets searched
Because "33s" is the target people arrive with, and the 15-inch 33 is the size the phrase was coined around. Someone measuring their Bronco's factory tire against that idea is measuring against 33x12.50R15 whether they realise it or not.
The practical answer for the vehicle is a 17-inch or 18-inch 33, which reaches the same 33.00 in with a much shorter sidewall.
What the numbers say if you did it
Revolutions per mile would drop from 671.0 to 611.2, and the gearing conversion returns exactly 4.10:1 for a truck starting at 3.73:1 — one of the rare cases where the arithmetic lands precisely on a set that is manufactured, rather than in the gap between two of them.
Sixty rotations in a mile is the plain version of that. It is worth holding on to because it is the quantity the vehicle itself works in: wheel-speed sensors count teeth on a ring, and every derived reading downstream of them is a rotation count wearing different units. The percentage is a convenience for people; the count is what the electronics have.
Ride height, with the wheel-size caveat
Half the diameter change is 1.47 in, and that is the axle-centre rise. It is unaffected by the wheel being smaller, because ride height is a function of the tire's outside diameter and nothing else. The wheel diameter changes where the sidewall sits, not where the ground is.
The comparison that is actually available
255/70R16 against 265/70R17 is the change this platform's owners can genuinely make, and it covers about half the distance.