-2.61%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 305/55R20, fitting 285/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.0 km/h | 25 mph | 24 mph |
| 60 km/h | 58.4 km/h | 37 mph | 36 mph |
| 80 km/h | 77.9 km/h | 50 mph | 48 mph |
| 100 km/h | 97.4 km/h | 62 mph | 61 mph |
| 120 km/h | 116.9 km/h | 75 mph | 73 mph |
Full comparison
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
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
-2.61%Safe swap — speedometer error is minor
Original
Overall diameter843.5 mm · 33.21 in
Sidewall height167.8 mm
Circumference2,649.9 mm
Revolutions / mile607.3
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 -2.61%. 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 km/h | 25 mph | 24 mph |
| 60 km/h | 58.4 km/h | 37 mph | 36 mph |
| 80 km/h | 77.9 km/h | 50 mph | 48 mph |
| 100 km/h | 97.4 km/h | 62 mph | 61 mph |
| 120 km/h | 116.9 km/h | 75 mph | 73 mph |
See individual specs: 305/55R20 · 285/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.