265/60R18 vs 285/70R17 — dropping a wheel size to gain two inches of tire
The Colorado ZR2 comparison, and the clearest case in this set of a smaller wheel carrying a much bigger tire.
285/70R17 is 2.19 in taller than 265/60R18 (7.17%) and 0.79 in wider. A speedometer calibrated for 265/60R18 would under-read by 7.17%.
Measured against each other
Overall diameter
30.52 in32.71 in+2.19 in (+7.17%)Section width
10.43 in / 265 mm11.22 in / 285 mm+0.79 in (+7.55%)Sidewall height
6.26 in7.85 in+1.59 inCircumference
95.9 in102.8 in+6.9 inRevolutions per mile
660.8616.6-44.2Ride height at the axle
——+1.09 inSpeedometer reading
——+7.17% under-read
| Dimension | 265/60R18 | 285/70R17 | Change |
|---|---|---|---|
| Overall diameter | 30.52 in | 32.71 in | +2.19 in (+7.17%) |
| Section width | 10.43 in / 265 mm | 11.22 in / 285 mm | +0.79 in (+7.55%) |
| Sidewall height | 6.26 in | 7.85 in | +1.59 in |
| Circumference | 95.9 in | 102.8 in | +6.9 in |
| Revolutions per mile | 660.8 | 616.6 | -44.2 |
| Ride height at the axle | — | — | +1.09 in |
| Speedometer reading | — | — | +7.17% under-read |
Calibrated for 265/60R18, running 285/70R17
- Dial reads 30
- 32.2 mph
- Dial reads 50
- 53.6 mph
- Dial reads 70
- 75.0 mph
- Odometer counts 100
- 107.2 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
An 18-inch wheel with a 30.52 in tire on it, against a 17-inch wheel with a 32.71 in tire. The smaller wheel wins by more than two inches.
The sidewall is doing all of it and then some
More than a quarter is added to the sidewall — 6.26 in against 7.85 in. That is 1.59 in per side, or 3.18 in of diameter, from which the inch of lost wheel is subtracted, leaving 2.19 in.
Both changes in the tire push the same way: 20 mm of extra width, and ten points of extra aspect ratio. Neither offsets the other, which is why the total is as large as it is.
Seven per cent is a real step
Hold engine speed and the axle wants 4.00:1 in place of 3.73:1. What is sold is 3.91:1, and very few mid-size trucks get re-geared for either figure, so the seven per cent is simply carried — and a thousand miles on the trip meter are 1,072 on the ground, which is the form of it that accumulates rather than the form of it you watch.
At this magnitude the change is noticeable in how the truck pulls away and in what the trip computer reports.
Why the off-road trim uses the smaller wheel
Because sidewall is the resource an 18-inch wheel does not leave room for. A tire designed to run at reduced pressure over rock needs a tall sidewall to deform without unseating, and 7.85 in of it on a 17-inch wheel is available where the 18-inch package's 6.26 in is not.
The 20 mm of extra width — 0.79 in, 7.55% — is a secondary consideration by comparison.
The trade
A 17-inch wheel and a taller tire is a heavier assembly than the 18-inch package in most combinations, and the extra 2.19 in of diameter has to fit somewhere. On this platform the constraint is usually the front arch liner and the upper control arm, measured at full lock and full compression rather than derived from a size code.
The middle option
265/60R18 against 275/70R17 makes the same wheel change with a narrower, shorter tire — 1.64 in instead of 2.19 in — which is the version that asks less of the truck.