235/45R18 vs 255/70R18
New overall diameter
32.06″
was 26.33″
True speed at an indicated 60
73.1 mph
speedometer reads low
Revolutions per mile
629
was 766 · −137
Switching from 235/45R18 to 255/70R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 235/45R18 | 255/70R18 | Change |
|---|---|---|---|
| Overall diameter | 26.33″ | 32.06″ | +5.73″ (+21.8%) |
| Sidewall height | 4.16″ (105.8 mm) | 7.03″ (178.5 mm) | +72.8 mm |
| Tread width | 235 mm | 255 mm | +20 mm |
| Circumference | 82.71″ | 100.70″ | +18.00″ |
| Revs per mile | 766 | 629 | −137 |
| Speedometer error | baseline | +21.8% | reads low |
Diameter overlay
235/45R18 255/70R18
Speedometer check
With 255/70R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
30.4 mph actual
35 mph shown
42.6 mph actual
45 mph shown
54.8 mph actual
55 mph shown
67.0 mph actual
65 mph shown
79.1 mph actual
75 mph shown
91.3 mph actual
Common questions
- Will 255/70R18 fit instead of 235/45R18?
- Not recommended. A 21.8% 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 32.06″, a change of +21.8%. 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 255/70R18?
- At an indicated 60 mph you would actually be travelling about 73.1 mph — a +21.8% 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 255/70R18?
- 235/45R18 turns roughly 766 revolutions per mile; 255/70R18 turns roughly 629. That is a change of −137 revolutions per mile, which is what shifts odometer and speedometer readings.