255/65R18 vs 255/55R20
New overall diameter
31.04″
was 31.05″
True speed at an indicated 60
60.0 mph
no meaningful change
Revolutions per mile
650
was 650 · +0
Switching from 255/65R18 to 255/55R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 255/65R18 | 255/55R20 | Change |
|---|---|---|---|
| Overall diameter | 31.05″ | 31.04″ | −0.01″ (−0.0%) |
| Sidewall height | 6.53″ (165.8 mm) | 5.52″ (140.3 mm) | −25.5 mm |
| Tread width | 255 mm | 255 mm | +0 mm |
| Circumference | 97.55″ | 97.53″ | −0.02″ |
| Revs per mile | 650 | 650 | +0 |
| Speedometer error | baseline | −0.0% | no change |
Diameter overlay
255/65R18 255/55R20
Speedometer check
With 255/55R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.0 mph actual
35 mph shown
35.0 mph actual
45 mph shown
45.0 mph actual
55 mph shown
55.0 mph actual
65 mph shown
65.0 mph actual
75 mph shown
75.0 mph actual
Common questions
- Will 255/55R20 fit instead of 255/65R18?
- Good fit. Within the 3% rule — overall diameter changes only 0.0%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Speedometer accuracy is essentially unchanged. Overall diameter goes from 31.05″ to 31.04″, a change of −0.0%. 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 60.0 mph — a −0.0% error. That is effectively no change.
- What is the difference in revolutions per mile between 255/65R18 and 255/55R20?
- 255/65R18 turns roughly 650 revolutions per mile; 255/55R20 turns roughly 650. That is a change of +0 revolutions per mile, which is what shifts odometer and speedometer readings.