-3.91%Borderline — noticeable speedometer/odometer error
Speedometer error
With a speedometer calibrated to 305/55R20, fitting 275/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 38.4 km/h | 25 mph | 24 mph |
| 60 km/h | 57.7 km/h | 37 mph | 36 mph |
| 80 km/h | 76.9 km/h | 50 mph | 48 mph |
| 100 km/h | 96.1 km/h | 62 mph | 60 mph |
| 120 km/h | 115.3 km/h | 75 mph | 72 mph |
Full comparison
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
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
-3.91%Borderline — noticeable speedometer/odometer error
Original
Overall diameter843.5 mm · 33.21 in
Sidewall height167.8 mm
Circumference2,649.9 mm
Revolutions / mile607.3
New
Overall diameter810.5 mm · 31.91 in
Sidewall height151.3 mm
Circumference2,546.3 mm
Revolutions / mile632
Speedometer & odometer error
The new tire changes overall diameter by -3.91%. 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 | 38.4 km/h | 25 mph | 24 mph |
| 60 km/h | 57.7 km/h | 37 mph | 36 mph |
| 80 km/h | 76.9 km/h | 50 mph | 48 mph |
| 100 km/h | 96.1 km/h | 62 mph | 60 mph |
| 120 km/h | 115.3 km/h | 75 mph | 72 mph |
See individual specs: 305/55R20 · 275/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.