315/70R17 vs 285/65R18
New overall diameter
32.59″
was 34.36″
True speed at an indicated 60
56.9 mph
your speedo reads high
Revolutions per mile
619
was 587, +32
| Measurement | 315/70R17 | 285/65R18 | Change |
|---|---|---|---|
| Overall diameter | 34.36″ | 32.59″ | −1.78″ (−5.2%) |
| Sidewall height | 8.68″ (220.5 mm) | 7.29″ (185.3 mm) | −35.3 mm |
| Tread width | 315 mm | 285 mm | −30 mm |
| Circumference | 107.95″ | 102.37″ | −5.58″ |
| Revs per mile | 587 | 619 | +32 |
| Speedometer error | baseline | −5.2% | reads high |
Diameter overlay
315/70R17 285/65R18
Speedometer check
With 285/65R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.7 mph actual
35 mph shown
33.2 mph actual
45 mph shown
42.7 mph actual
55 mph shown
52.2 mph actual
65 mph shown
61.6 mph actual
75 mph shown
71.1 mph actual
Common questions
- Will 285/65R18 fit instead of 315/70R17?
- 5.2% is past the 5% mark, which is where this stops being a tire choice and starts being a project. Expect rubbing, meaningful speedometer error, and extra load on ABS, stability control and the drivetrain. Recalibration is the usual fix. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 34.36″ to 32.59″, a change of −5.2%. 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 285/65R18?
- At an indicated 60 mph you are really doing about 56.9 mph, a −5.2% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 315/70R17 and 285/65R18?
- 315/70R17 turns about 587 times per mile and 285/65R18 turns about 619, a difference of +32. Your speedometer and odometer both count rotations, so this number is what throws them off.