-2.73%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 305/40R22, fitting 275/45R21 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 38.9 km/h | 25 mph | 24 mph |
| 60 km/h | 58.4 km/h | 37 mph | 36 mph |
| 80 km/h | 77.8 km/h | 50 mph | 48 mph |
| 100 km/h | 97.3 km/h | 62 mph | 60 mph |
| 120 km/h | 116.7 km/h | 75 mph | 73 mph |
Full comparison
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
275/45R21
Overall diameter780.9 mm · 30.74 in
Section width275 mm · 10.83 in
Sidewall height123.8 mm · 4.87 in
Circumference2453.3 mm · 96.6 in
Revolutions per mile656
Revolutions per km408
-2.73%Safe swap — speedometer error is minor
Original
Overall diameter802.8 mm · 31.61 in
Sidewall height122 mm
Circumference2,522.1 mm
Revolutions / mile638.1
New
Overall diameter780.9 mm · 30.74 in
Sidewall height123.8 mm
Circumference2,453.3 mm
Revolutions / mile656
Speedometer & odometer error
The new tire changes overall diameter by -2.73%. 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 | 38.9 km/h | 25 mph | 24 mph |
| 60 km/h | 58.4 km/h | 37 mph | 36 mph |
| 80 km/h | 77.8 km/h | 50 mph | 48 mph |
| 100 km/h | 97.3 km/h | 62 mph | 60 mph |
| 120 km/h | 116.7 km/h | 75 mph | 73 mph |
See individual specs: 305/40R22 · 275/45R21. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.