ElpiAuto285/55R20 vs 305/55R20
305/55R20 is 2.7% taller (22 mm) than 285/55R20 — 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 — 285/55R20
- 285section width11.2″ across the tread, sidewall to sidewall
- 55aspect ratiosidewall is 55% of the width — 6.2″ tall
- Rconstructionradial — standard on every modern road car
- 20wheel diametermounts on a 20-inch wheel
In flotation sizing: ≈ 32.3x11.2R20
Size B — 305/55R20
- 305section width12.0″ across the tread, sidewall to sidewall
- 55aspect ratiosidewall is 55% of the width — 6.6″ tall
- Rconstructionradial — standard on every modern road car
- 20wheel diametermounts on a 20-inch wheel
In flotation sizing: ≈ 33.2x12R20
285/55R20 vs 305/55R20, measurement by measurement
Every dimension in inches, with the difference and which size is bigger
| Measurement | 285/55R20 | 305/55R20 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 32.34″ | 33.21″ | +0.87″ | +2.7% |
| Section width | 11.22″ | 12.01″ | +0.79″ | +7.0% |
| Sidewall height | 6.17″ | 6.60″ | +0.43″ | +7.0% |
| Circumference | 101.61″ | 104.33″ | +2.72″ | +2.7% |
| Revs per mile | 623.6 | 607.3 | −16.3 | −2.6% |
| Revs per km | 387.5 | 377.4 | −10.1 | −2.6% |
| Approved rim width estimate | 8.5–11″ | 9–11.5″ | — | — |
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 285/55R20 to 305/55R20. 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 61.6 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 under-counts by roughly 261 miles.
| Speedometer reads | True speed on 305/55R20 | Error |
|---|---|---|
| 20 mph | 20.5 mph | +0.5 |
| 30 mph | 30.8 mph | +0.8 |
| 40 mph | 41.1 mph | +1.1 |
| 50 mph | 51.3 mph | +1.3 |
| 60 mph | 61.6 mph | +1.6 |
| 70 mph | 71.9 mph | +1.9 |
| 80 mph | 82.1 mph | +2.1 |
With 285/55R20 as the original size. A taller tire makes the dial read low — the gap grows with speed.
Reading this comparison
The overall diameter is what matters here: 305/55R20 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.