+1.63%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 285/45R21, fitting 305/40R22 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61.0 km/h | 37 mph | 38 mph |
| 80 km/h | 81.3 km/h | 50 mph | 51 mph |
| 100 km/h | 101.6 km/h | 62 mph | 63 mph |
| 120 km/h | 122.0 km/h | 75 mph | 76 mph |
Full comparison
285/45R21
Overall diameter789.9 mm · 31.10 in
Section width285 mm · 11.22 in
Sidewall height128.3 mm · 5.05 in
Circumference2481.5 mm · 97.7 in
Revolutions per mile649
Revolutions per km403
305/40R22
Overall diameter802.8 mm · 31.61 in
Section width305 mm · 12.01 in
Sidewall height122.0 mm · 4.80 in
Circumference2522.1 mm · 99.3 in
Revolutions per mile638
Revolutions per km396
+1.63%Safe swap — speedometer error is minor
Original
Overall diameter789.9 mm · 31.1 in
Sidewall height128.3 mm
Circumference2,481.5 mm
Revolutions / mile648.5
New
Overall diameter802.8 mm · 31.61 in
Sidewall height122 mm
Circumference2,522.1 mm
Revolutions / mile638.1
Speedometer & odometer error
The new tire changes overall diameter by +1.63%. 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 | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61 km/h | 37 mph | 38 mph |
| 80 km/h | 81.3 km/h | 50 mph | 51 mph |
| 100 km/h | 101.6 km/h | 62 mph | 63 mph |
| 120 km/h | 122 km/h | 75 mph | 76 mph |
See individual specs: 285/45R21 · 305/40R22. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.