235/45R18 vs 235/55R18
New overall diameter
28.18″
was 26.33″
True speed at an indicated 60
64.2 mph
speedometer reads low
Revolutions per mile
716
was 766 · −50
Switching from 235/45R18 to 235/55R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 235/45R18 | 235/55R18 | Change |
|---|---|---|---|
| Overall diameter | 26.33″ | 28.18″ | +1.85″ (+7.0%) |
| Sidewall height | 4.16″ (105.8 mm) | 5.09″ (129.3 mm) | +23.5 mm |
| Tread width | 235 mm | 235 mm | +0 mm |
| Circumference | 82.71″ | 88.52″ | +5.81″ |
| Revs per mile | 766 | 716 | −50 |
| Speedometer error | baseline | +7.0% | reads low |
Diameter overlay
235/45R18 235/55R18
Speedometer check
With 235/55R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.8 mph actual
35 mph shown
37.5 mph actual
45 mph shown
48.2 mph actual
55 mph shown
58.9 mph actual
65 mph shown
69.6 mph actual
75 mph shown
80.3 mph actual
Common questions
- Will 235/55R18 fit instead of 235/45R18?
- Not recommended. A 7.0% 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 lower than your true speed. Overall diameter goes from 26.33″ to 28.18″, a change of +7.0%. 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 235/55R18?
- At an indicated 60 mph you would actually be travelling about 64.2 mph — a +7.0% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 235/45R18 and 235/55R18?
- 235/45R18 turns roughly 766 revolutions per mile; 235/55R18 turns roughly 716. That is a change of −50 revolutions per mile, which is what shifts odometer and speedometer readings.