305/55R20 vs 285/65R20
New overall diameter
34.59″
was 33.21″
True speed at an indicated 60
62.5 mph
your speedo reads low
Revolutions per mile
583
was 607, −24
| Measurement | 305/55R20 | 285/65R20 | Change |
|---|---|---|---|
| Overall diameter | 33.21″ | 34.59″ | +1.38″ (+4.1%) |
| Sidewall height | 6.60″ (167.8 mm) | 7.29″ (185.3 mm) | +17.5 mm |
| Tread width | 305 mm | 285 mm | −20 mm |
| Circumference | 104.33″ | 108.66″ | +4.33″ |
| Revs per mile | 607 | 583 | −24 |
| Speedometer error | baseline | +4.1% | reads low |
Diameter overlay
305/55R20 285/65R20
Speedometer check
With 285/65R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.0 mph actual
35 mph shown
36.5 mph actual
45 mph shown
46.9 mph actual
55 mph shown
57.3 mph actual
65 mph shown
67.7 mph actual
75 mph shown
78.1 mph actual
Common questions
- Will 285/65R20 fit instead of 305/55R20?
- Overall diameter changes 4.1%; section width changes -20 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 33.21″ to 34.59″, a change of +4.1%. 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 285/65R20?
- At an indicated 60 mph you are really doing about 62.5 mph, a +4.1% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 305/55R20 and 285/65R20?
- 305/55R20 turns about 607 times per mile and 285/65R20 turns about 583, a difference of −24. Your speedometer and odometer both count rotations, so this number is what throws them off.