255/50R20 vs 235/65R18
New overall diameter
30.03″
was 30.04″
True speed at an indicated 60
60.0 mph
no meaningful change
Revolutions per mile
672
was 671 · +0
Switching from 255/50R20 to 235/65R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 255/50R20 | 235/65R18 | Change |
|---|---|---|---|
| Overall diameter | 30.04″ | 30.03″ | −0.01″ (−0.0%) |
| Sidewall height | 5.02″ (127.5 mm) | 6.01″ (152.8 mm) | +25.3 mm |
| Tread width | 255 mm | 235 mm | −20 mm |
| Circumference | 94.37″ | 94.33″ | −0.04″ |
| Revs per mile | 671 | 672 | +0 |
| Speedometer error | baseline | −0.0% | no change |
Diameter overlay
255/50R20 235/65R18
Speedometer check
With 235/65R18 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 235/65R18 fit instead of 255/50R20?
- 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 30.04″ to 30.03″, 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 235/65R18?
- 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 255/50R20 and 235/65R18?
- 255/50R20 turns roughly 671 revolutions per mile; 235/65R18 turns roughly 672. That is a change of +0 revolutions per mile, which is what shifts odometer and speedometer readings.