245/75R17 vs 255/75R17
New overall diameter
32.06″
was 31.47″
True speed at an indicated 60
61.1 mph
speedometer reads low
Revolutions per mile
629
was 641 · −12
Switching from 245/75R17 to 255/75R17? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/75R17 | 255/75R17 | Change |
|---|---|---|---|
| Overall diameter | 31.47″ | 32.06″ | +0.59″ (+1.9%) |
| Sidewall height | 7.23″ (183.8 mm) | 7.53″ (191.3 mm) | +7.5 mm |
| Tread width | 245 mm | 255 mm | +10 mm |
| Circumference | 98.86″ | 100.72″ | +1.86″ |
| Revs per mile | 641 | 629 | −12 |
| Speedometer error | baseline | +1.9% | reads low |
Diameter overlay
245/75R17 255/75R17
Speedometer check
With 255/75R17 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
45.8 mph actual
55 mph shown
56.0 mph actual
65 mph shown
66.2 mph actual
75 mph shown
76.4 mph actual
Common questions
- Will 255/75R17 fit instead of 245/75R17?
- Good fit. Within the 3% rule — overall diameter changes only 1.9%. 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 31.47″ to 32.06″, a change of +1.9%. 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 255/75R17?
- At an indicated 60 mph you would actually be travelling about 61.1 mph — a +1.9% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 245/75R17 and 255/75R17?
- 245/75R17 turns roughly 641 revolutions per mile; 255/75R17 turns roughly 629. That is a change of −12 revolutions per mile, which is what shifts odometer and speedometer readings.