275/65R18 vs 285/65R20 — two inches of wheel and half an inch of tire
A wheel-and-tire package sold as one decision, and an attempt to work out which half of it does what.
285/65R20 is 2.51 in taller than 275/65R18 (7.83%) and 0.39 in wider. A speedometer calibrated for 275/65R18 would under-read by 7.83%.
Measured against each other
Overall diameter
32.07 in34.59 in+2.51 in (+7.83%)Section width
10.83 in / 275 mm11.22 in / 285 mm+0.39 in (+3.64%)Sidewall height
7.04 in7.29 in+0.26 inCircumference
100.8 in108.7 in+7.9 inRevolutions per mile
628.8583.1-45.7Ride height at the axle
——+1.26 inSpeedometer reading
——+7.83% under-read
| Dimension | 275/65R18 | 285/65R20 | Change |
|---|---|---|---|
| Overall diameter | 32.07 in | 34.59 in | +2.51 in (+7.83%) |
| Section width | 10.83 in / 275 mm | 11.22 in / 285 mm | +0.39 in (+3.64%) |
| Sidewall height | 7.04 in | 7.29 in | +0.26 in |
| Circumference | 100.8 in | 108.7 in | +7.9 in |
| Revolutions per mile | 628.8 | 583.1 | -45.7 |
| Ride height at the axle | — | — | +1.26 in |
| Speedometer reading | — | — | +7.83% under-read |
Calibrated for 275/65R18, running 285/65R20
- Dial reads 30
- 32.3 mph
- Dial reads 50
- 53.9 mph
- Dial reads 70
- 75.5 mph
- Odometer counts 100
- 107.8 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Most of the extra height here is wheel. Almost none of it is tire, and the sidewall barely moves at all.
Splitting the change
Sidewall goes from 7.04 in to 7.29 in — a quarter of an inch. The wheel goes from 18 in to 20 in — two inches. Add two sidewalls' worth of the first to the second and you get 2.51 in of extra diameter.
So of the 2.51 in gained, exactly 2.00 in is the wheel and 0.51 in is the tire. Anyone evaluating this package as a tire upgrade is misattributing four-fifths of the effect.
What it costs and what it does not
Section width barely participates: 10.83 in to 11.22 in, 0.39 in, 3.64%. This is a height change almost purely, and height is what the speedometer charges for — 7.83%, or better than twice the proportional change in width.
Note where the axle gain comes from. Only a quarter of an inch of it is the tire; the rest is the wheel, which contributes an inch of radius and no compliance at all.
Why it is priced as one decision
Because it is sold as one. Wheel-and-tire packages are quoted as a single figure and fitted in a single visit, and the buyer's mental model is "bigger tires". Separating the two makes the proposition clearer: two-thirds of the money is going on wheels, and four-fifths of the height gain is coming from them.
An owner who wanted the height alone could get most of it far more cheaply by staying on 18s and moving to a taller tire.
The 18-inch alternative
275/65R18 against 285/75R18 reaches 34.83 in — a quarter of an inch taller than this package — on the wheels the truck already has, with an 8.42 in sidewall instead of a 7.29 in one. Comparing the two pages is the fastest way to see what a 20-inch wheel costs in compliance.
Two platforms, one package
This combination is sold to fourteenth-generation F-150 owners and to Ram 1500 owners in almost the same terms, because both trucks are delivered on 18-inch wheels with 275/65R18 and both have 20-inch factory-style wheels available with a 285/65R20 on them. The arithmetic does not distinguish between the two vehicles; what does distinguish them is offset and the shape of the wheel well.
What is not settled
Whether the 20-inch wheel clears, what its offset does to the track, and what load index the tire carries. The placard states the requirement; the arithmetic above states the geometry.