ElpiAuto275/55R20 vs 285/50R20
285/50R20 is 2.2% shorter (17 mm) than 275/55R20 — right at the edge of the ±3% swap band.
Two sizes
Also accepts 35X12.50R17 and 31x10.5-15.
Both sizes, drawn to scale
1 grid = 1 inWhat each number means
The two size codes broken down — which digit is which, and what it does on the car.
Size A — 275/55R20
- 275section width10.8″ across the tread, sidewall to sidewall
- 55aspect ratiosidewall is 55% of the width — 6.0″ tall
- Rconstructionradial — standard on every modern road car
- 20wheel diametermounts on a 20-inch wheel
In flotation sizing: ≈ 31.9x10.8R20
Size B — 285/50R20
- 285section width11.2″ across the tread, sidewall to sidewall
- 50aspect ratiosidewall is 50% of the width — 5.6″ tall
- Rconstructionradial — standard on every modern road car
- 20wheel diametermounts on a 20-inch wheel
In flotation sizing: ≈ 31.2x11.2R20
275/55R20 vs 285/50R20, measurement by measurement
Every dimension in inches, with the difference and which size is bigger
| Measurement | 275/55R20 | 285/50R20 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 31.91″ | 31.22″ | −0.69″ | −2.2% |
| Section width | 10.83″ | 11.22″ | +0.39″ | +3.6% |
| Sidewall height | 5.95″ | 5.61″ | −0.34″ | −5.8% |
| Circumference | 100.25″ | 98.08″ | −2.16″ | −2.2% |
| Revs per mile | 632.0 | 646.0 | +13.9 | +2.2% |
| Revs per km | 392.7 | 401.4 | +8.7 | +2.2% |
| Approved rim width estimate | 8–10.5″ | 8.5–11″ | — | — |
Overall diameter is the controlling number — clearance, gearing, speedometer accuracy and odometer reading all follow from it. Revs per mile/km is what your ABS, speedometer and odometer actually count.
What changes on the road
Direction and rough size of each effect going from 275/55R20 to 285/50R20. Geometry only — real tires vary by brand and compound.
Speedometer & odometer error
What changes on the dash when the overall diameter changes.
A speedometer doesn't measure distance. It counts how fast the wheels turn and multiplies by a fixed factory number for tire circumference. Change the tire and that factory number is now wrong.
Going to a bigger tire, each turn covers more ground, so at a true 58.7 mph the wheels spin at the rate the factory calls 60. The dial reads low — you are speeding without knowing it. A smaller tire does the opposite, and the odometer also over-counts miles you didn't drive.
Over 10,000 indicated miles, this change means the odometer over-counts by roughly 221 miles.
| Speedometer reads | True speed on 285/50R20 | Error |
|---|---|---|
| 20 mph | 19.6 mph | −0.4 |
| 30 mph | 29.4 mph | −0.6 |
| 40 mph | 39.1 mph | −0.9 |
| 50 mph | 48.9 mph | −1.1 |
| 60 mph | 58.7 mph | −1.3 |
| 70 mph | 68.5 mph | −1.5 |
| 80 mph | 78.3 mph | −1.7 |
With 275/55R20 as the original size. A shorter tire makes the dial read high — the gap grows with speed.
Reading this comparison
The overall diameter is what matters here: 275/55R20 is the taller tire. It sets ride height, gearing and how far the tire travels per revolution — which is what the speedometer and odometer count. Width and sidewall change fit and feel (a wider tire needs a wider wheel and more clearance; a shorter sidewall sharpens steering) but not the gearing.
Figures are geometric estimates from the size codes; real moulded tires vary by brand by 1–3%. Confirm load rating, wheel width and physical clearance on your vehicle before fitting. To compare any other two sizes, use the tire size comparison tool; to decode one size and see what else fits its wheel, the tire size calculator.