245/40R18 vs 245/45R17
New overall diameter
25.68″
was 25.72″
True speed at an indicated 60
59.9 mph
speedometer reads high
Revolutions per mile
785
was 784 · +1
Switching from 245/40R18 to 245/45R17? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/40R18 | 245/45R17 | Change |
|---|---|---|---|
| Overall diameter | 25.72″ | 25.68″ | −0.04″ (−0.1%) |
| Sidewall height | 3.86″ (98.0 mm) | 4.34″ (110.3 mm) | +12.3 mm |
| Tread width | 245 mm | 245 mm | +0 mm |
| Circumference | 80.79″ | 80.68″ | −0.11″ |
| Revs per mile | 784 | 785 | +1 |
| Speedometer error | baseline | −0.1% | reads high |
Diameter overlay
245/40R18 245/45R17
Speedometer check
With 245/45R17 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.0 mph actual
35 mph shown
35.0 mph actual
45 mph shown
44.9 mph actual
55 mph shown
54.9 mph actual
65 mph shown
64.9 mph actual
75 mph shown
74.9 mph actual
Common questions
- Will 245/45R17 fit instead of 245/40R18?
- Good fit. Within the 3% rule — overall diameter changes only 0.1%. 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 25.72″ to 25.68″, a change of −0.1%. 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 245/45R17?
- At an indicated 60 mph you would actually be travelling about 59.9 mph — a −0.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 245/40R18 and 245/45R17?
- 245/40R18 turns roughly 784 revolutions per mile; 245/45R17 turns roughly 785. That is a change of +1 revolutions per mile, which is what shifts odometer and speedometer readings.