-3.65%Borderline — noticeable speedometer/odometer error
Speedometer error
With a speedometer calibrated to 275/55R20, fitting 275/45R21 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 38.5 km/h | 25 mph | 24 mph |
| 60 km/h | 57.8 km/h | 37 mph | 36 mph |
| 80 km/h | 77.1 km/h | 50 mph | 48 mph |
| 100 km/h | 96.3 km/h | 62 mph | 60 mph |
| 120 km/h | 115.6 km/h | 75 mph | 72 mph |
Full comparison
275/55R20
Overall diameter810.5 mm · 31.91 in
Section width275 mm · 10.83 in
Sidewall height151.3 mm · 5.95 in
Circumference2546.3 mm · 100.2 in
Revolutions per mile632
Revolutions per km393
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
-3.65%Borderline — noticeable speedometer/odometer error
Original
Overall diameter810.5 mm · 31.91 in
Sidewall height151.3 mm
Circumference2,546.3 mm
Revolutions / mile632
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 -3.65%. 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 | 38.5 km/h | 25 mph | 24 mph |
| 60 km/h | 57.8 km/h | 37 mph | 36 mph |
| 80 km/h | 77.1 km/h | 50 mph | 48 mph |
| 100 km/h | 96.3 km/h | 62 mph | 60 mph |
| 120 km/h | 115.6 km/h | 75 mph | 72 mph |
See individual specs: 275/55R20 · 275/45R21. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.