-0.76%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 285/60R20, fitting 305/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.7 km/h | 25 mph | 25 mph |
| 60 km/h | 59.5 km/h | 37 mph | 37 mph |
| 80 km/h | 79.4 km/h | 50 mph | 49 mph |
| 100 km/h | 99.2 km/h | 62 mph | 62 mph |
| 120 km/h | 119.1 km/h | 75 mph | 74 mph |
Full comparison
285/60R20
Overall diameter850.0 mm · 33.46 in
Section width285 mm · 11.22 in
Sidewall height171.0 mm · 6.73 in
Circumference2670.4 mm · 105.1 in
Revolutions per mile603
Revolutions per km374
305/55R20
Overall diameter843.5 mm · 33.21 in
Section width305 mm · 12.01 in
Sidewall height167.8 mm · 6.60 in
Circumference2649.9 mm · 104.3 in
Revolutions per mile607
Revolutions per km377
-0.76%Safe swap — speedometer error is minor
Original
Overall diameter850 mm · 33.46 in
Sidewall height171 mm
Circumference2,670.4 mm
Revolutions / mile602.7
New
Overall diameter843.5 mm · 33.21 in
Sidewall height167.8 mm
Circumference2,649.9 mm
Revolutions / mile607.3
Speedometer & odometer error
The new tire changes overall diameter by -0.76%. 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 | 39.7 km/h | 25 mph | 25 mph |
| 60 km/h | 59.5 km/h | 37 mph | 37 mph |
| 80 km/h | 79.4 km/h | 50 mph | 49 mph |
| 100 km/h | 99.2 km/h | 62 mph | 62 mph |
| 120 km/h | 119.1 km/h | 75 mph | 74 mph |
See individual specs: 285/60R20 · 305/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.