285/60R20 vs 305/55R20
New overall diameter
33.21″
was 33.46″
True speed at an indicated 60
59.5 mph
speedometer reads high
Revolutions per mile
607
was 603 · +5
Switching from 285/60R20 to 305/55R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/60R20 | 305/55R20 | Change |
|---|---|---|---|
| Overall diameter | 33.46″ | 33.21″ | −0.26″ (−0.8%) |
| Sidewall height | 6.73″ (171.0 mm) | 6.60″ (167.8 mm) | −3.3 mm |
| Tread width | 285 mm | 305 mm | +20 mm |
| Circumference | 105.13″ | 104.33″ | −0.80″ |
| Revs per mile | 603 | 607 | +5 |
| Speedometer error | baseline | −0.8% | reads high |
Diameter overlay
285/60R20 305/55R20
Speedometer check
With 305/55R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.8 mph actual
35 mph shown
34.7 mph actual
45 mph shown
44.7 mph actual
55 mph shown
54.6 mph actual
65 mph shown
64.5 mph actual
75 mph shown
74.4 mph actual
Common questions
- Will 305/55R20 fit instead of 285/60R20?
- Good fit. Within the 3% rule — overall diameter changes only 0.8%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read higher than your true speed. Overall diameter goes from 33.46″ to 33.21″, a change of −0.8%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 305/55R20?
- At an indicated 60 mph you would actually be travelling about 59.5 mph — a −0.8% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/60R20 and 305/55R20?
- 285/60R20 turns roughly 603 revolutions per mile; 305/55R20 turns roughly 607. That is a change of +5 revolutions per mile, which is what shifts odometer and speedometer readings.