+1.36%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 275/55R20, fitting 285/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.8 km/h | 37 mph | 38 mph |
| 80 km/h | 81.1 km/h | 50 mph | 50 mph |
| 100 km/h | 101.4 km/h | 62 mph | 63 mph |
| 120 km/h | 121.6 km/h | 75 mph | 76 mph |
Full comparison
275/55R20
Overall diameter810.5 mm · 31.91 in
Section width275 mm · 10.83 in
Sidewall height151.3 mm · 5.95 in
Circumference2546.3 mm · 100.2 in
Revolutions per mile632
Revolutions per km393
285/55R20
Overall diameter821.5 mm · 32.34 in
Section width285 mm · 11.22 in
Sidewall height156.8 mm · 6.17 in
Circumference2580.8 mm · 101.6 in
Revolutions per mile624
Revolutions per km387
+1.36%Safe swap — speedometer error is minor
Original
Overall diameter810.5 mm · 31.91 in
Sidewall height151.3 mm
Circumference2,546.3 mm
Revolutions / mile632
New
Overall diameter821.5 mm · 32.34 in
Sidewall height156.8 mm
Circumference2,580.8 mm
Revolutions / mile623.6
Speedometer & odometer error
The new tire changes overall diameter by +1.36%. With a speedometer calibrated to the original tire, that same percentage is how far the speedometer and odometer read off — a taller tire means your true speed is higher than indicated and the odometer under-counts distance.
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.8 km/h | 37 mph | 38 mph |
| 80 km/h | 81.1 km/h | 50 mph | 50 mph |
| 100 km/h | 101.4 km/h | 62 mph | 63 mph |
| 120 km/h | 121.6 km/h | 75 mph | 76 mph |
See individual specs: 275/55R20 · 285/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.