285/40R22 vs 255/55R20
New overall diameter
31.04″
was 30.98″
True speed at an indicated 60
60.1 mph
speedometer reads low
Revolutions per mile
650
was 651 · −1
Switching from 285/40R22 to 255/55R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/40R22 | 255/55R20 | Change |
|---|---|---|---|
| Overall diameter | 30.98″ | 31.04″ | +0.07″ (+0.2%) |
| Sidewall height | 4.49″ (114.0 mm) | 5.52″ (140.3 mm) | +26.3 mm |
| Tread width | 285 mm | 255 mm | −30 mm |
| Circumference | 97.32″ | 97.53″ | +0.21″ |
| Revs per mile | 651 | 650 | −1 |
| Speedometer error | baseline | +0.2% | reads low |
Diameter overlay
285/40R22 255/55R20
Speedometer check
With 255/55R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.1 mph actual
55 mph shown
55.1 mph actual
65 mph shown
65.1 mph actual
75 mph shown
75.2 mph actual
Common questions
- Will 255/55R20 fit instead of 285/40R22?
- Good fit. Within the 3% rule — overall diameter changes only 0.2%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 30.98″ to 31.04″, a change of +0.2%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 255/55R20?
- At an indicated 60 mph you would actually be travelling about 60.1 mph — a +0.2% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 285/40R22 and 255/55R20?
- 285/40R22 turns roughly 651 revolutions per mile; 255/55R20 turns roughly 650. That is a change of −1 revolutions per mile, which is what shifts odometer and speedometer readings.