305/55R20 vs 285/60R20
New overall diameter
33.46″
was 33.21″
True speed at an indicated 60
60.5 mph
your speedo reads low
Revolutions per mile
603
was 607, −5
| Measurement | 305/55R20 | 285/60R20 | Change |
|---|---|---|---|
| Overall diameter | 33.21″ | 33.46″ | +0.26″ (+0.8%) |
| Sidewall height | 6.60″ (167.8 mm) | 6.73″ (171.0 mm) | +3.3 mm |
| Tread width | 305 mm | 285 mm | −20 mm |
| Circumference | 104.33″ | 105.13″ | +0.80″ |
| Revs per mile | 607 | 603 | −5 |
| Speedometer error | baseline | +0.8% | reads low |
Diameter overlay
305/55R20 285/60R20
Speedometer check
With 285/60R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.2 mph actual
35 mph shown
35.3 mph actual
45 mph shown
45.3 mph actual
55 mph shown
55.4 mph actual
65 mph shown
65.5 mph actual
75 mph shown
75.6 mph actual
Common questions
- Will 285/60R20 fit instead of 305/55R20?
- Diameter moves 0.8%, 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 low, so you are going faster than it says. Overall diameter goes from 33.21″ to 33.46″, a change of +0.8%. 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 285/60R20?
- At an indicated 60 mph you are really doing about 60.5 mph, a +0.8% 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 285/60R20?
- 305/55R20 turns about 607 times per mile and 285/60R20 turns about 603, a difference of −5. Your speedometer and odometer both count rotations, so this number is what throws them off.