285/65R20 vs 285/70R18
New overall diameter
33.71″
was 34.59″
True speed at an indicated 60
58.5 mph
your speedo reads high
Revolutions per mile
598
was 583, +15
| Measurement | 285/65R20 | 285/70R18 | Change |
|---|---|---|---|
| Overall diameter | 34.59″ | 33.71″ | −0.88″ (−2.5%) |
| Sidewall height | 7.29″ (185.3 mm) | 7.85″ (199.5 mm) | +14.3 mm |
| Tread width | 285 mm | 285 mm | +0 mm |
| Circumference | 108.66″ | 105.90″ | −2.76″ |
| Revs per mile | 583 | 598 | +15 |
| Speedometer error | baseline | −2.5% | reads high |
Diameter overlay
285/65R20 285/70R18
Speedometer check
With 285/70R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.4 mph actual
35 mph shown
34.1 mph actual
45 mph shown
43.9 mph actual
55 mph shown
53.6 mph actual
65 mph shown
63.4 mph actual
75 mph shown
73.1 mph actual
Common questions
- Will 285/70R18 fit instead of 285/65R20?
- Overall diameter changes 2.5%; section width changes 0 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional. Overall diameter goes from 34.59″ to 33.71″, a change of −2.5%. 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 285/70R18?
- At an indicated 60 mph you are really doing about 58.5 mph, a −2.5% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/65R20 and 285/70R18?
- 285/65R20 turns about 583 times per mile and 285/70R18 turns about 598, a difference of +15. Your speedometer and odometer both count rotations, so this number is what throws them off.