245/75R17 vs 255/65R17
New overall diameter
30.05″
was 31.47″
True speed at an indicated 60
57.3 mph
speedometer reads high
Revolutions per mile
671
was 641 · +30
Switching from 245/75R17 to 255/65R17? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/75R17 | 255/65R17 | Change |
|---|---|---|---|
| Overall diameter | 31.47″ | 30.05″ | −1.42″ (−4.5%) |
| Sidewall height | 7.23″ (183.8 mm) | 6.53″ (165.8 mm) | −18.0 mm |
| Tread width | 245 mm | 255 mm | +10 mm |
| Circumference | 98.86″ | 94.41″ | −4.45″ |
| Revs per mile | 641 | 671 | +30 |
| Speedometer error | baseline | −4.5% | reads high |
Diameter overlay
245/75R17 255/65R17
Speedometer check
With 255/65R17 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.9 mph actual
35 mph shown
33.4 mph actual
45 mph shown
43.0 mph actual
55 mph shown
52.5 mph actual
65 mph shown
62.1 mph actual
75 mph shown
71.6 mph actual
Common questions
- Will 255/65R17 fit instead of 245/75R17?
- Marginal fit — proceed with care. The 4.5% 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 31.47″ to 30.05″, a change of −4.5%. 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 255/65R17?
- At an indicated 60 mph you would actually be travelling about 57.3 mph — a −4.5% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 245/75R17 and 255/65R17?
- 245/75R17 turns roughly 641 revolutions per mile; 255/65R17 turns roughly 671. That is a change of +30 revolutions per mile, which is what shifts odometer and speedometer readings.