305/70R17 vs 305/60R18
New overall diameter
32.41″
was 33.81″
True speed at an indicated 60
57.5 mph
speedometer reads high
Revolutions per mile
622
was 596 · +26
Switching from 305/70R17 to 305/60R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 305/70R17 | 305/60R18 | Change |
|---|---|---|---|
| Overall diameter | 33.81″ | 32.41″ | −1.40″ (−4.1%) |
| Sidewall height | 8.41″ (213.5 mm) | 7.20″ (183.0 mm) | −30.5 mm |
| Tread width | 305 mm | 305 mm | +0 mm |
| Circumference | 106.22″ | 101.82″ | −4.40″ |
| Revs per mile | 596 | 622 | +26 |
| Speedometer error | baseline | −4.1% | reads high |
Diameter overlay
305/70R17 305/60R18
Speedometer check
With 305/60R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.0 mph actual
35 mph shown
33.5 mph actual
45 mph shown
43.1 mph actual
55 mph shown
52.7 mph actual
65 mph shown
62.3 mph actual
75 mph shown
71.9 mph actual
Common questions
- Will 305/60R18 fit instead of 305/70R17?
- Marginal fit — proceed with care. The 4.1% 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 higher than your true speed. Overall diameter goes from 33.81″ to 32.41″, a change of −4.1%. 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 305/60R18?
- At an indicated 60 mph you would actually be travelling about 57.5 mph — a −4.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 305/70R17 and 305/60R18?
- 305/70R17 turns roughly 596 revolutions per mile; 305/60R18 turns roughly 622. That is a change of +26 revolutions per mile, which is what shifts odometer and speedometer readings.