285/70R18 vs 275/65R18
New overall diameter
32.07″
was 33.71″
True speed at an indicated 60
57.1 mph
speedometer reads high
Revolutions per mile
629
was 598 · +30
Switching from 285/70R18 to 275/65R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/70R18 | 275/65R18 | Change |
|---|---|---|---|
| Overall diameter | 33.71″ | 32.07″ | −1.63″ (−4.8%) |
| Sidewall height | 7.85″ (199.5 mm) | 7.04″ (178.8 mm) | −20.8 mm |
| Tread width | 285 mm | 275 mm | −10 mm |
| Circumference | 105.90″ | 100.77″ | −5.13″ |
| Revs per mile | 598 | 629 | +30 |
| Speedometer error | baseline | −4.8% | reads high |
Diameter overlay
285/70R18 275/65R18
Speedometer check
With 275/65R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.8 mph actual
35 mph shown
33.3 mph actual
45 mph shown
42.8 mph actual
55 mph shown
52.3 mph actual
65 mph shown
61.8 mph actual
75 mph shown
71.4 mph actual
Common questions
- Will 275/65R18 fit instead of 285/70R18?
- Marginal fit — proceed with care. The 4.8% 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 33.71″ to 32.07″, a change of −4.8%. 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 275/65R18?
- At an indicated 60 mph you would actually be travelling about 57.1 mph — a −4.8% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/70R18 and 275/65R18?
- 285/70R18 turns roughly 598 revolutions per mile; 275/65R18 turns roughly 629. That is a change of +30 revolutions per mile, which is what shifts odometer and speedometer readings.