-2.54%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 275/65R20, fitting 305/55R20 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 | 58.5 km/h | 37 mph | 36 mph |
| 80 km/h | 78.0 km/h | 50 mph | 48 mph |
| 100 km/h | 97.5 km/h | 62 mph | 61 mph |
| 120 km/h | 116.9 km/h | 75 mph | 73 mph |
Full comparison
275/65R20
Overall diameter865.5 mm · 34.07 in
Section width275 mm · 10.83 in
Sidewall height178.8 mm · 7.04 in
Circumference2719.0 mm · 107.0 in
Revolutions per mile592
Revolutions per km368
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
-2.54%Safe swap — speedometer error is minor
Original
Overall diameter865.5 mm · 34.07 in
Sidewall height178.8 mm
Circumference2,719 mm
Revolutions / mile591.9
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 -2.54%. 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 | 58.5 km/h | 37 mph | 36 mph |
| 80 km/h | 78 km/h | 50 mph | 48 mph |
| 100 km/h | 97.5 km/h | 62 mph | 61 mph |
| 120 km/h | 116.9 km/h | 75 mph | 73 mph |
See individual specs: 275/65R20 · 305/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.