185/65R15 vs 175/65R15
New overall diameter
23.96″
was 24.47″
True speed at an indicated 60
58.7 mph
speedometer reads high
Revolutions per mile
842
was 824 · +18
Switching from 185/65R15 to 175/65R15? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 185/65R15 | 175/65R15 | Change |
|---|---|---|---|
| Overall diameter | 24.47″ | 23.96″ | −0.51″ (−2.1%) |
| Sidewall height | 4.73″ (120.3 mm) | 4.48″ (113.8 mm) | −6.5 mm |
| Tread width | 185 mm | 175 mm | −10 mm |
| Circumference | 76.87″ | 75.26″ | −1.61″ |
| Revs per mile | 824 | 842 | +18 |
| Speedometer error | baseline | −2.1% | reads high |
Diameter overlay
185/65R15 175/65R15
Speedometer check
With 175/65R15 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.5 mph actual
35 mph shown
34.3 mph actual
45 mph shown
44.1 mph actual
55 mph shown
53.8 mph actual
65 mph shown
63.6 mph actual
75 mph shown
73.4 mph actual
Common questions
- Will 175/65R15 fit instead of 185/65R15?
- Good fit. Within the 3% rule — overall diameter changes only 2.1%. 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 24.47″ to 23.96″, a change of −2.1%. 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 175/65R15?
- At an indicated 60 mph you would actually be travelling about 58.7 mph — a −2.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 185/65R15 and 175/65R15?
- 185/65R15 turns roughly 824 revolutions per mile; 175/65R15 turns roughly 842. That is a change of +18 revolutions per mile, which is what shifts odometer and speedometer readings.