245/60R18 vs 235/55R18
New overall diameter
28.18″
was 29.57″
True speed at an indicated 60
57.2 mph
speedometer reads high
Revolutions per mile
716
was 682 · +34
Switching from 245/60R18 to 235/55R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/60R18 | 235/55R18 | Change |
|---|---|---|---|
| Overall diameter | 29.57″ | 28.18″ | −1.40″ (−4.7%) |
| Sidewall height | 5.79″ (147.0 mm) | 5.09″ (129.3 mm) | −17.8 mm |
| Tread width | 245 mm | 235 mm | −10 mm |
| Circumference | 92.91″ | 88.52″ | −4.39″ |
| Revs per mile | 682 | 716 | +34 |
| Speedometer error | baseline | −4.7% | reads high |
Diameter overlay
245/60R18 235/55R18
Speedometer check
With 235/55R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.8 mph actual
35 mph shown
33.3 mph actual
45 mph shown
42.9 mph actual
55 mph shown
52.4 mph actual
65 mph shown
61.9 mph actual
75 mph shown
71.5 mph actual
Common questions
- Will 235/55R18 fit instead of 245/60R18?
- Marginal fit — proceed with care. The 4.7% diameter change is between 3% and 5%. It can work, but expect noticeable speedometer error and check wheel-well clearance, scrub radius and gearing before committing. Your speedometer will read higher than your true speed. Overall diameter goes from 29.57″ to 28.18″, a change of −4.7%. 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 235/55R18?
- At an indicated 60 mph you would actually be travelling about 57.2 mph — a −4.7% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 245/60R18 and 235/55R18?
- 245/60R18 turns roughly 682 revolutions per mile; 235/55R18 turns roughly 716. That is a change of +34 revolutions per mile, which is what shifts odometer and speedometer readings.