305/60R18 vs 285/70R18
New overall diameter
33.71″
was 32.41″
True speed at an indicated 60
62.4 mph
speedometer reads low
Revolutions per mile
598
was 622 · −24
Switching from 305/60R18 to 285/70R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 305/60R18 | 285/70R18 | Change |
|---|---|---|---|
| Overall diameter | 32.41″ | 33.71″ | +1.30″ (+4.0%) |
| Sidewall height | 7.20″ (183.0 mm) | 7.85″ (199.5 mm) | +16.5 mm |
| Tread width | 305 mm | 285 mm | −20 mm |
| Circumference | 101.82″ | 105.90″ | +4.08″ |
| Revs per mile | 622 | 598 | −24 |
| Speedometer error | baseline | +4.0% | reads low |
Diameter overlay
305/60R18 285/70R18
Speedometer check
With 285/70R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.0 mph actual
35 mph shown
36.4 mph actual
45 mph shown
46.8 mph actual
55 mph shown
57.2 mph actual
65 mph shown
67.6 mph actual
75 mph shown
78.0 mph actual
Common questions
- Will 285/70R18 fit instead of 305/60R18?
- Marginal fit — proceed with care. The 4.0% 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 lower than your true speed. Overall diameter goes from 32.41″ to 33.71″, a change of +4.0%. 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 285/70R18?
- At an indicated 60 mph you would actually be travelling about 62.4 mph — a +4.0% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 305/60R18 and 285/70R18?
- 305/60R18 turns roughly 622 revolutions per mile; 285/70R18 turns roughly 598. That is a change of −24 revolutions per mile, which is what shifts odometer and speedometer readings.