255/70R17 vs 285/70R17 — 30 mm on the Silverado's smaller wheel
The Trail Boss question for owners on 17-inch wheels, and three width increments taken in one move.
285/70R17 is 1.65 in taller than 255/70R17 (5.32%) and 1.18 in wider. A speedometer calibrated for 255/70R17 would under-read by 5.32%.
Measured against each other
Overall diameter
31.06 in32.71 in+1.65 in (+5.32%)Section width
10.04 in / 255 mm11.22 in / 285 mm+1.18 in (+11.76%)Sidewall height
7.03 in7.85 in+0.83 inCircumference
97.6 in102.8 in+5.2 inRevolutions per mile
649.4616.6-32.8Ride height at the axle
——+0.83 inSpeedometer reading
——+5.32% under-read
| Dimension | 255/70R17 | 285/70R17 | Change |
|---|---|---|---|
| Overall diameter | 31.06 in | 32.71 in | +1.65 in (+5.32%) |
| Section width | 10.04 in / 255 mm | 11.22 in / 285 mm | +1.18 in (+11.76%) |
| Sidewall height | 7.03 in | 7.85 in | +0.83 in |
| Circumference | 97.6 in | 102.8 in | +5.2 in |
| Revolutions per mile | 649.4 | 616.6 | -32.8 |
| Ride height at the axle | — | — | +0.83 in |
| Speedometer reading | — | — | +5.32% under-read |
Calibrated for 255/70R17, running 285/70R17
- Dial reads 30
- 31.6 mph
- Dial reads 50
- 52.7 mph
- Dial reads 70
- 73.7 mph
- Odometer counts 100
- 105.3 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Three width increments at once, with the aspect ratio untouched. That combination produces the largest fixed-profile step in this comparison set.
Three steps, taken together
255 to 265 to 275 to 285: the metric scale moves in tens, and this pairing skips two intermediate rungs. The casing grows 1.18 in, from 10.04 in to 11.22 in, and because the sidewall is fixed at 70% of whatever it is given, it grows too — 7.03 in becomes 7.85 in.
Overall diameter: 31.06 in to 32.71 in, an increase of 1.65 in, or 5.32%.
The gearing coincidence
A truck on 3.73:1 would want 3.93:1 to hold engine speed after this swap, and the 3.91:1 set sits almost exactly on top of that — two hundredths away, which is as near as a catalogue ratio ever gets to an arbitrary requirement. Close enough to be tempting, and almost nobody does it, because five per cent is not enough to make anything on a half-ton mechanically unhappy.
What is left to live with is a needle light by 3.7 mph at a dial reading of 70.
The 17-inch wheel is the point
Silverados are sold across 17, 18, 20 and 22-inch wheels, and the 17 is the one that leaves room for a tall sidewall. 7.85 in of it on the 285 is more than any 20-inch fitment on the same truck can offer, which is why owners planning larger tires often start by finding a set of 17s.
The trade is appearance: a 17-inch wheel under a modern full-size pickup looks small, and that is the reason the factory fits bigger ones.
Where the width lands
1.18 in of extra casing, 11.76%. On this generation the interaction is with the front wheel-well liner and the lower control arm rather than the fender lip, and wheel offset determines how much of the extra width goes inboard. That is a measurement taken with the wheels turned, not a figure derived from a size code.
The same destination from a different start
Owners on 265/70R17 rather than 255/70R17 face a smaller version of the same step — 1.10 in instead of 1.65 in, from 20 mm of width rather than 30.