-9.35%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 325/65R20, fitting 305/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 36.3 km/h | 25 mph | 23 mph |
| 60 km/h | 54.4 km/h | 37 mph | 34 mph |
| 80 km/h | 72.5 km/h | 50 mph | 45 mph |
| 100 km/h | 90.7 km/h | 62 mph | 56 mph |
| 120 km/h | 108.8 km/h | 75 mph | 68 mph |
Full comparison
325/65R20
Overall diameter930.5 mm · 36.63 in
Section width325 mm · 12.80 in
Sidewall height211.3 mm · 8.32 in
Circumference2923.3 mm · 115.1 in
Revolutions per mile551
Revolutions per km342
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
-9.35%Not recommended — recalibrate the speedometer
Original
Overall diameter930.5 mm · 36.63 in
Sidewall height211.3 mm
Circumference2,923.3 mm
Revolutions / mile550.5
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 -9.35%. 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 | 36.3 km/h | 25 mph | 23 mph |
| 60 km/h | 54.4 km/h | 37 mph | 34 mph |
| 80 km/h | 72.5 km/h | 50 mph | 45 mph |
| 100 km/h | 90.7 km/h | 62 mph | 56 mph |
| 120 km/h | 108.8 km/h | 75 mph | 68 mph |
See individual specs: 325/65R20 · 305/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.