275/45R21 — the only 21-inch size in this catalogue
The largest wheel on the option sheet carries the smallest tire on the option sheet.
Metric notation · Crossover and SUV
- Overall diameter
- 30.74 in
- 781 mm
- Section width
- 10.83 in
- 275 mm
- Sidewall height
- 4.87 in
- 124 mm
- Aspect ratio
- 45%
- sidewall as a share of width
- Wheel diameter
- 21 in
- the hole in the middle
- Circumference
- 96.6 in
- 2453 mm
- Revolutions per mile
- 656.0
- geometric, not measured
- Revolutions per km
- 407.6
- geometric, not measured
- Typical load index
- 98–116
- 1653–2756 lb per tire
Crossovers are heavier than the cars they share size families with, so the same nominal size is produced at a higher index for SUV fitment than for passenger fitment. Two tires wearing the same size code can carry materially different loads.
The load-index range shown is the band this CLASS of size is typically produced in — not a list of what is in stock for this exact size today. Load index and speed symbol are printed on the sidewall of the specific tire you are holding, and the required minimum is on the placard of the specific vehicle you are fitting. Read both.
Nearest real sizes in imperial notation, same wheel
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Platforms recorded with this size
Twenty-one inches is a wheel diameter almost nobody builds tires for in volume, and this catalogue holds exactly one size on it. The Ford Explorer's sixth generation is the vehicle that puts it there.
The Explorer's three factory sizes, measured
| Size | Wheel | Diameter | Section width | Sidewall |
|---|---|---|---|---|
| 255/65R18 | 18 in | 31.05 in | 10.04 in | 6.53 in |
| 255/55R20 | 20 in | 31.04 in | 10.04 in | 5.52 in |
| 275/45R21 | 21 in | 30.74 in | 10.83 in | 4.87 in |
The 18-inch and 20-inch options agree on rolling diameter to a hundredth of an inch, which is ordinary practice — one calibration serves both. The 21-inch option does not join them. It is 0.31 in shorter than the other two, and it is the only one of the three that changes the section width, adding 0.79 in.
So the top wheel option on this vehicle is simultaneously the widest tire and the shortest, and whoever specified it accepted a rolling diameter about one per cent adrift of the rest of the range rather than build a taller 21-inch casing.
What 4.87 inches of sidewall is doing here
Subtract the wheel and halve the remainder: this tire puts 4.87 in of casing between the rim and the road. Every other size the Explorer is recorded with carries at least 5.52 in, and the 18-inch option carries 6.53 in.
That is the whole content of the 21-inch decision. The vehicle sits at very nearly the same height, the drivetrain sees very nearly the same tire, and the difference bought is a third of an inch less compliant structure on each corner in exchange for two inches of visible rim. The plus sizing reference covers the trade in general terms.
Where it lands against tires on ordinary wheels
Nothing on a 21-inch wheel can be compared with anything else on a 21-inch wheel, so the comparisons have to go sideways. 265/70R16 stands 30.61 in, 0.14 in under this size, on 7.30 in of sidewall. 265/65R17 is 30.56 in, 265/60R18 is 30.52 in, and 245/70R17 is 30.50 in.
Four mid-size truck and SUV codes, all within a quarter of an inch of a 21-inch crossover tire, and all with between one and a half and two and a half inches more casing depth.
Class: Crossover and SUV. Every figure above is computed from the size code, not stored — see how these numbers are calculated.