235/55R18 vs 245/45R18
New overall diameter
26.68″
was 28.18″
True speed at an indicated 60
56.8 mph
speedometer reads high
Revolutions per mile
756
was 716 · +40
Switching from 235/55R18 to 245/45R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 235/55R18 | 245/45R18 | Change |
|---|---|---|---|
| Overall diameter | 28.18″ | 26.68″ | −1.50″ (−5.3%) |
| Sidewall height | 5.09″ (129.3 mm) | 4.34″ (110.3 mm) | −19.0 mm |
| Tread width | 235 mm | 245 mm | +10 mm |
| Circumference | 88.52″ | 83.82″ | −4.70″ |
| Revs per mile | 716 | 756 | +40 |
| Speedometer error | baseline | −5.3% | reads high |
Diameter overlay
235/55R18 245/45R18
Speedometer check
With 245/45R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.7 mph actual
35 mph shown
33.1 mph actual
45 mph shown
42.6 mph actual
55 mph shown
52.1 mph actual
65 mph shown
61.5 mph actual
75 mph shown
71.0 mph actual
Common questions
- Will 245/45R18 fit instead of 235/55R18?
- Not recommended. A 5.3% diameter change exceeds the 5% rule. Expect significant speedometer error, potential clearance and rubbing issues, and stress on ABS, stability control and drivetrain components. Your speedometer will read higher than your true speed. Overall diameter goes from 28.18″ to 26.68″, a change of −5.3%. 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 245/45R18?
- At an indicated 60 mph you would actually be travelling about 56.8 mph — a −5.3% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 235/55R18 and 245/45R18?
- 235/55R18 turns roughly 716 revolutions per mile; 245/45R18 turns roughly 756. That is a change of +40 revolutions per mile, which is what shifts odometer and speedometer readings.