255/40R18 vs 235/40R18
New overall diameter
25.40″
was 26.03″
True speed at an indicated 60
58.5 mph
speedometer reads high
Revolutions per mile
794
was 775 · +19
Switching from 255/40R18 to 235/40R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 255/40R18 | 235/40R18 | Change |
|---|---|---|---|
| Overall diameter | 26.03″ | 25.40″ | −0.63″ (−2.4%) |
| Sidewall height | 4.02″ (102.0 mm) | 3.70″ (94.0 mm) | −8.0 mm |
| Tread width | 255 mm | 235 mm | −20 mm |
| Circumference | 81.78″ | 79.80″ | −1.98″ |
| Revs per mile | 775 | 794 | +19 |
| Speedometer error | baseline | −2.4% | reads high |
Diameter overlay
255/40R18 235/40R18
Speedometer check
With 235/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.4 mph actual
35 mph shown
34.2 mph actual
45 mph shown
43.9 mph actual
55 mph shown
53.7 mph actual
65 mph shown
63.4 mph actual
75 mph shown
73.2 mph actual
Common questions
- Will 235/40R18 fit instead of 255/40R18?
- Good fit. Within the 3% rule — overall diameter changes only 2.4%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read higher than your true speed. Overall diameter goes from 26.03″ to 25.40″, a change of −2.4%. 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 235/40R18?
- At an indicated 60 mph you would actually be travelling about 58.5 mph — a −2.4% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 255/40R18 and 235/40R18?
- 255/40R18 turns roughly 775 revolutions per mile; 235/40R18 turns roughly 794. That is a change of +19 revolutions per mile, which is what shifts odometer and speedometer readings.