255/45R20 vs 235/60R18
New overall diameter
29.10″
was 29.04″
True speed at an indicated 60
60.1 mph
your speedo reads low
Revolutions per mile
693
was 695, −2
| Measurement | 255/45R20 | 235/60R18 | Change |
|---|---|---|---|
| Overall diameter | 29.04″ | 29.10″ | +0.07″ (+0.2%) |
| Sidewall height | 4.52″ (114.8 mm) | 5.55″ (141.0 mm) | +26.3 mm |
| Tread width | 255 mm | 235 mm | −20 mm |
| Circumference | 91.22″ | 91.43″ | +0.21″ |
| Revs per mile | 695 | 693 | −2 |
| Speedometer error | baseline | +0.2% | reads low |
Diameter overlay
255/45R20 235/60R18
Speedometer check
With 235/60R18 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 235/60R18 fit instead of 255/45R20?
- Diameter moves 0.2%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 29.04″ to 29.10″, a change of +0.2%. 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 235/60R18?
- At an indicated 60 mph you are really doing 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 255/45R20 and 235/60R18?
- 255/45R20 turns about 695 times per mile and 235/60R18 turns about 693, a difference of −2. Your speedometer and odometer both count rotations, so this number is what throws them off.