235/40R18 vs 235/45R18
New overall diameter
26.33″
was 25.40″
True speed at an indicated 60
62.2 mph
speedometer reads low
Revolutions per mile
766
was 794 · −28
Switching from 235/40R18 to 235/45R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 235/40R18 | 235/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.40″ | 26.33″ | +0.93″ (+3.6%) |
| Sidewall height | 3.70″ (94.0 mm) | 4.16″ (105.8 mm) | +11.8 mm |
| Tread width | 235 mm | 235 mm | +0 mm |
| Circumference | 79.80″ | 82.71″ | +2.91″ |
| Revs per mile | 794 | 766 | −28 |
| Speedometer error | baseline | +3.6% | reads low |
Diameter overlay
235/40R18 235/45R18
Speedometer check
With 235/45R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.9 mph actual
35 mph shown
36.3 mph actual
45 mph shown
46.6 mph actual
55 mph shown
57.0 mph actual
65 mph shown
67.4 mph actual
75 mph shown
77.7 mph actual
Common questions
- Will 235/45R18 fit instead of 235/40R18?
- Marginal fit — proceed with care. The 3.6% 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 lower than your true speed. Overall diameter goes from 25.40″ to 26.33″, a change of +3.6%. 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 235/45R18?
- At an indicated 60 mph you would actually be travelling about 62.2 mph — a +3.6% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 235/40R18 and 235/45R18?
- 235/40R18 turns roughly 794 revolutions per mile; 235/45R18 turns roughly 766. That is a change of −28 revolutions per mile, which is what shifts odometer and speedometer readings.