275/55R20 vs 285/65R20 — the metric route to a 34-inch tire
A third-generation Silverado's factory 20s taken as far as metric sizes will take them.
285/65R20 is 2.68 in taller than 275/55R20 (8.39%) and 0.39 in wider. A speedometer calibrated for 275/55R20 would under-read by 8.39%.
Measured against each other
Overall diameter
31.91 in34.59 in+2.68 in (+8.39%)Section width
10.83 in / 275 mm11.22 in / 285 mm+0.39 in (+3.64%)Sidewall height
5.95 in7.29 in+1.34 inCircumference
100.2 in108.7 in+8.4 inRevolutions per mile
632.0583.1-48.9Ride height at the axle
——+1.34 inSpeedometer reading
——+8.39% under-read
| Dimension | 275/55R20 | 285/65R20 | Change |
|---|---|---|---|
| Overall diameter | 31.91 in | 34.59 in | +2.68 in (+8.39%) |
| Section width | 10.83 in / 275 mm | 11.22 in / 285 mm | +0.39 in (+3.64%) |
| Sidewall height | 5.95 in | 7.29 in | +1.34 in |
| Circumference | 100.2 in | 108.7 in | +8.4 in |
| Revolutions per mile | 632.0 | 583.1 | -48.9 |
| Ride height at the axle | — | — | +1.34 in |
| Speedometer reading | — | — | +8.39% under-read |
Calibrated for 275/55R20, running 285/65R20
- Dial reads 30
- 32.5 mph
- Dial reads 50
- 54.2 mph
- Dial reads 70
- 75.9 mph
- Odometer counts 100
- 108.4 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Ten points of aspect ratio is two steps of profile, and on a wide casing that is worth a great deal more than the 10 mm of width that accompanies it here.
Where 2.68 in comes from
1.34 in is added per side, taking the sidewall from 5.95 in to 7.29 in. Almost all of that is the profile change; the extra 10 mm of casing contributes only a small fraction.
Doubled, 1.34 in becomes the 2.68 in of extra diameter, taking the tire from 31.91 in to 34.59 in. The section width moves 0.39 in, from 10.83 in to 11.22 in, which is 3.64%.
Eight per cent is a large step for a factory wheel
In gearing terms: 3.73:1 converts to 4.04:1 against an available 4.10:1, and 4.10:1 converts to 4.44:1 against an available 4.56:1. Neither lands close, which is a hint that a change this large on a half-ton is usually carried rather than corrected.
Carried, in practice, means living with the readings in the panel above rather than paying to move them.
What makes this route attractive
No flotation tire is involved. 285/65R20 is a metric size available in mainstream highway and all-terrain patterns, and it reaches 34.59 in on 11.22 in of nominal section — 0.39 in wider than the 10.83 in the truck left the factory on. The flotation route to a similar height asks for 12.50 in, which is 1.67 in of extra casing rather than four-tenths.
That ratio is the argument, and it is arithmetic. What either casing needs from a wheel is a figure on the tire manufacturer's specification sheet, not something derivable from a size code, and this site does not carry it.
For an owner who wants height rather than stance, that is the cleaner path — and 34.59 in is only 0.41 in short of a nominal 35.
Where the arch comes in
Because the width barely moved, this step binds above the tire rather than beside it — which is the unusual part. On a full-size pickup with factory suspension, 2.68 in of extra diameter is about the largest change taken without other work, and all of the argument is vertical: the sidewall gain, the axle gain and the lost headroom above the crown are the same 1.34 in appearing three times.
The intermediate size
275/55R20 against 275/60R20 is the first half of this move on its own — one profile step, 1.08 in — for anyone who wants to take it in stages.