265/70R17 vs 33x12.50R20 — a wheel and a tire, priced as one
An older Ram's 17-inch package against a modern 20-inch one, where the taller assembly has the shorter sidewall.
33x12.50R20 is 1.39 in taller than 265/70R17 (4.41%) and 2.07 in wider. A speedometer calibrated for 265/70R17 would under-read by 4.41%.
Measured against each other
Overall diameter
31.61 in33.00 in+1.39 in (+4.41%)Section width
10.43 in / 265 mm12.50 in / 318 mm+2.07 in (+19.81%)Sidewall height
7.30 in6.50 in-0.80 inCircumference
99.3 in103.7 in+4.4 inRevolutions per mile
638.1611.2-26.9Ride height at the axle
——+0.70 inSpeedometer reading
——+4.41% under-read
| Dimension | 265/70R17 | 33x12.50R20 | Change |
|---|---|---|---|
| Overall diameter | 31.61 in | 33.00 in | +1.39 in (+4.41%) |
| Section width | 10.43 in / 265 mm | 12.50 in / 318 mm | +2.07 in (+19.81%) |
| Sidewall height | 7.30 in | 6.50 in | -0.80 in |
| Circumference | 99.3 in | 103.7 in | +4.4 in |
| Revolutions per mile | 638.1 | 611.2 | -26.9 |
| Ride height at the axle | — | — | +0.70 in |
| Speedometer reading | — | — | +4.41% under-read |
Calibrated for 265/70R17, running 33x12.50R20
- Dial reads 30
- 31.3 mph
- Dial reads 50
- 52.2 mph
- Dial reads 70
- 73.1 mph
- Odometer counts 100
- 104.4 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
The finished assembly is 1.39 in taller and has 0.80 in less rubber in it. That combination is only possible because the wheel changed at the same time as the tire.
Where the sidewall went
The 17-inch fitment has 7.30 in of sidewall. The 20-inch fitment has 6.50 in. Three inches of extra wheel radius — an inch and a half at each end — has been paid for out of sidewall and then some, and the 1.39 in of extra overall height comes from the wheel rather than from the tire.
That is the arithmetic behind the whole 20-inch wheel package: buy diameter, sell compliance.
What the vehicle registers
4.41% of speedometer error, and the axle centre 0.70 in higher. Expressed as the vehicle sees it, the wheel now completes 611.2 rotations per mile rather than 638.1, and every thousand miles the odometer records are 1,044 covered.
Those figures are identical to the 17-inch flotation version of the same nominal size, because a 33 is 33.00 in regardless of what wheel it is wrapped around. The wheel diameter changes the sidewall and nothing the speedometer can see.
Width is where the money goes
10.43 in to 12.50 in — 2.07 in, or 19.81%. A 12.50 in section on a 20-inch wheel is a large object and the wheel underneath it is heavy. Unsprung mass rises substantially, and no size code carries that figure.
Two variables, one price tag
Because these packages are sold as wheel-and-tire sets, the buyer usually evaluates them as a single decision and never separates the wheel's contribution from the tire's. Separating them is worthwhile: everything gained in height came from the wheel, everything lost in sidewall came from the wheel, and the tire's own contribution to the change is its width.
The aspect ratio collapses
The metric tire states a 70-series profile. The 20-inch flotation tire works out to a 52-series — 6.50 in of sidewall on a 12.50 in casing. Eighteen series apart, on tires whose heights differ by less than an inch and a half. Nothing on the flotation tire prints that number, which is why buyers comparing "a 33" against a factory tire so rarely notice how different an object they are looking at.
The 17-inch version of the same tire
265/70R17 against 33x12.50R17 reaches the identical 33.00 in with an 8.00 in sidewall instead of a 6.50 in one, on the wheels the truck already has. The two pages have the same diameter arithmetic and completely different consequences.