205/65R15 vs 185/65R15
New overall diameter
24.47″
was 25.49″
True speed at an indicated 60
57.6 mph
speedometer reads high
Revolutions per mile
824
was 791 · +33
Switching from 205/65R15 to 185/65R15? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 205/65R15 | 185/65R15 | Change |
|---|---|---|---|
| Overall diameter | 25.49″ | 24.47″ | −1.02″ (−4.0%) |
| Sidewall height | 5.25″ (133.3 mm) | 4.73″ (120.3 mm) | −13.0 mm |
| Tread width | 205 mm | 185 mm | −20 mm |
| Circumference | 80.09″ | 76.87″ | −3.22″ |
| Revs per mile | 791 | 824 | +33 |
| Speedometer error | baseline | −4.0% | reads high |
Diameter overlay
205/65R15 185/65R15
Speedometer check
With 185/65R15 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.0 mph actual
35 mph shown
33.6 mph actual
45 mph shown
43.2 mph actual
55 mph shown
52.8 mph actual
65 mph shown
62.4 mph actual
75 mph shown
72.0 mph actual
Common questions
- Will 185/65R15 fit instead of 205/65R15?
- 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 higher than your true speed. Overall diameter goes from 25.49″ to 24.47″, 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 185/65R15?
- At an indicated 60 mph you would actually be travelling about 57.6 mph — a −4.0% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 205/65R15 and 185/65R15?
- 205/65R15 turns roughly 791 revolutions per mile; 185/65R15 turns roughly 824. That is a change of +33 revolutions per mile, which is what shifts odometer and speedometer readings.