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
| 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?
- Diameter moves 0.0%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer stays accurate. Overall diameter goes from 30.04″ to 30.03″, a change of −0.0%. The rule of thumb most shops use: under 3% is fine, 3 to 5% needs a clearance check, over 5% needs recalibration.
- How far off will my speedometer read on 235/65R18?
- At an indicated 60 mph you are really doing 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 about 671 times per mile and 235/65R18 turns about 672, a difference of +0. Your speedometer and odometer both count rotations, so this number is what throws them off.