+0.66%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 275/60R20, fitting 305/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.3 km/h | 25 mph | 25 mph |
| 60 km/h | 60.4 km/h | 37 mph | 38 mph |
| 80 km/h | 80.5 km/h | 50 mph | 50 mph |
| 100 km/h | 100.7 km/h | 62 mph | 63 mph |
| 120 km/h | 120.8 km/h | 75 mph | 75 mph |
Full comparison
275/60R20
Overall diameter838.0 mm · 32.99 in
Section width275 mm · 10.83 in
Sidewall height165.0 mm · 6.50 in
Circumference2632.7 mm · 103.6 in
Revolutions per mile611
Revolutions per km380
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
+0.66%Safe swap — speedometer error is minor
Original
Overall diameter838 mm · 32.99 in
Sidewall height165 mm
Circumference2,632.7 mm
Revolutions / mile611.3
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 +0.66%. 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 | 40.3 km/h | 25 mph | 25 mph |
| 60 km/h | 60.4 km/h | 37 mph | 38 mph |
| 80 km/h | 80.5 km/h | 50 mph | 50 mph |
| 100 km/h | 100.7 km/h | 62 mph | 63 mph |
| 120 km/h | 120.8 km/h | 75 mph | 75 mph |
See individual specs: 275/60R20 · 305/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.