235/80R17 vs 255/65R18
New overall diameter
31.05″
was 31.80″
True speed at an indicated 60
58.6 mph
your speedo reads high
Revolutions per mile
650
was 634, +15
| Measurement | 235/80R17 | 255/65R18 | Change |
|---|---|---|---|
| Overall diameter | 31.80″ | 31.05″ | −0.75″ (−2.4%) |
| Sidewall height | 7.40″ (188.0 mm) | 6.53″ (165.8 mm) | −22.3 mm |
| Tread width | 235 mm | 255 mm | +20 mm |
| Circumference | 99.91″ | 97.55″ | −2.36″ |
| Revs per mile | 634 | 650 | +15 |
| Speedometer error | baseline | −2.4% | reads high |
Diameter overlay
235/80R17 255/65R18
Speedometer check
With 255/65R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.4 mph actual
35 mph shown
34.2 mph actual
45 mph shown
43.9 mph actual
55 mph shown
53.7 mph actual
65 mph shown
63.5 mph actual
75 mph shown
73.2 mph actual
Common questions
- Will 255/65R18 fit instead of 235/80R17?
- Diameter moves 2.4%, 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 31.80″ to 31.05″, a change of −2.4%. 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 255/65R18?
- At an indicated 60 mph you are really doing about 58.6 mph, a −2.4% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 235/80R17 and 255/65R18?
- 235/80R17 turns about 634 times per mile and 255/65R18 turns about 650, a difference of +15. Your speedometer and odometer both count rotations, so this number is what throws them off.