+4.15%Borderline — noticeable speedometer/odometer error
Speedometer error
With a speedometer calibrated to 305/55R20, fitting 285/65R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 41.7 km/h | 25 mph | 26 mph |
| 60 km/h | 62.5 km/h | 37 mph | 39 mph |
| 80 km/h | 83.3 km/h | 50 mph | 52 mph |
| 100 km/h | 104.1 km/h | 62 mph | 65 mph |
| 120 km/h | 125.0 km/h | 75 mph | 78 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
285/65R20
Overall diameter878.5 mm · 34.59 in
Section width285 mm · 11.22 in
Sidewall height185.3 mm · 7.29 in
Circumference2759.9 mm · 108.7 in
Revolutions per mile583
Revolutions per km362
+4.15%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 diameter878.5 mm · 34.59 in
Sidewall height185.3 mm
Circumference2,759.9 mm
Revolutions / mile583.1
Speedometer & odometer error
The new tire changes overall diameter by +4.15%. 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 | 41.7 km/h | 25 mph | 26 mph |
| 60 km/h | 62.5 km/h | 37 mph | 39 mph |
| 80 km/h | 83.3 km/h | 50 mph | 52 mph |
| 100 km/h | 104.1 km/h | 62 mph | 65 mph |
| 120 km/h | 125 km/h | 75 mph | 78 mph |
See individual specs: 305/55R20 · 285/65R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.