175/65R15 vs 185/65R15
New overall diameter
24.47″
was 23.96″
True speed at an indicated 60
61.3 mph
speedometer reads low
Revolutions per mile
824
was 842 · −18
Switching from 175/65R15 to 185/65R15? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 175/65R15 | 185/65R15 | Change |
|---|---|---|---|
| Overall diameter | 23.96″ | 24.47″ | +0.51″ (+2.1%) |
| Sidewall height | 4.48″ (113.8 mm) | 4.73″ (120.3 mm) | +6.5 mm |
| Tread width | 175 mm | 185 mm | +10 mm |
| Circumference | 75.26″ | 76.87″ | +1.61″ |
| Revs per mile | 842 | 824 | −18 |
| Speedometer error | baseline | +2.1% | reads low |
Diameter overlay
175/65R15 185/65R15
Speedometer check
With 185/65R15 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.5 mph actual
35 mph shown
35.7 mph actual
45 mph shown
46.0 mph actual
55 mph shown
56.2 mph actual
65 mph shown
66.4 mph actual
75 mph shown
76.6 mph actual
Common questions
- Will 185/65R15 fit instead of 175/65R15?
- 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 lower than your true speed. Overall diameter goes from 23.96″ to 24.47″, 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 185/65R15?
- At an indicated 60 mph you would actually be travelling about 61.3 mph — a +2.1% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 175/65R15 and 185/65R15?
- 175/65R15 turns roughly 842 revolutions per mile; 185/65R15 turns roughly 824. That is a change of −18 revolutions per mile, which is what shifts odometer and speedometer readings.