255/70R17 vs 245/70R17
New overall diameter
30.50″
was 31.06″
True speed at an indicated 60
58.9 mph
your speedo reads high
Revolutions per mile
661
was 649, +12
| Measurement | 255/70R17 | 245/70R17 | Change |
|---|---|---|---|
| Overall diameter | 31.06″ | 30.50″ | −0.55″ (−1.8%) |
| Sidewall height | 7.03″ (178.5 mm) | 6.75″ (171.5 mm) | −7.0 mm |
| Tread width | 255 mm | 245 mm | −10 mm |
| Circumference | 97.56″ | 95.83″ | −1.73″ |
| Revs per mile | 649 | 661 | +12 |
| Speedometer error | baseline | −1.8% | reads high |
Diameter overlay
255/70R17 245/70R17
Speedometer check
With 245/70R17 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.6 mph actual
35 mph shown
34.4 mph actual
45 mph shown
44.2 mph actual
55 mph shown
54.0 mph actual
65 mph shown
63.8 mph actual
75 mph shown
73.7 mph actual
Common questions
- Will 245/70R17 fit instead of 255/70R17?
- Diameter moves 1.8%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 31.06″ to 30.50″, a change of −1.8%. The rule of thumb most shops use: under 3% is fine, 3 to 5% needs a clearance check, over 5% needs recalibration.
- How far off will my speedometer read on 245/70R17?
- At an indicated 60 mph you are really doing about 58.9 mph, a −1.8% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 255/70R17 and 245/70R17?
- 255/70R17 turns about 649 times per mile and 245/70R17 turns about 661, a difference of +12. Your speedometer and odometer both count rotations, so this number is what throws them off.