255/40R18 vs 245/45R18
New overall diameter
26.68″
was 26.03″
True speed at an indicated 60
61.5 mph
speedometer reads low
Revolutions per mile
756
was 775 · −19
Switching from 255/40R18 to 245/45R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 255/40R18 | 245/45R18 | Change |
|---|---|---|---|
| Overall diameter | 26.03″ | 26.68″ | +0.65″ (+2.5%) |
| Sidewall height | 4.02″ (102.0 mm) | 4.34″ (110.3 mm) | +8.3 mm |
| Tread width | 255 mm | 245 mm | −10 mm |
| Circumference | 81.78″ | 83.82″ | +2.04″ |
| Revs per mile | 775 | 756 | −19 |
| Speedometer error | baseline | +2.5% | reads low |
Diameter overlay
255/40R18 245/45R18
Speedometer check
With 245/45R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.6 mph actual
35 mph shown
35.9 mph actual
45 mph shown
46.1 mph actual
55 mph shown
56.4 mph actual
65 mph shown
66.6 mph actual
75 mph shown
76.9 mph actual
Common questions
- Will 245/45R18 fit instead of 255/40R18?
- Good fit. Within the 3% rule — overall diameter changes only 2.5%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 26.03″ to 26.68″, a change of +2.5%. 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 61.5 mph — a +2.5% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 255/40R18 and 245/45R18?
- 255/40R18 turns roughly 775 revolutions per mile; 245/45R18 turns roughly 756. That is a change of −19 revolutions per mile, which is what shifts odometer and speedometer readings.