285/55R20 vs 305/60R18
New overall diameter
32.41″
was 32.34″
True speed at an indicated 60
60.1 mph
speedometer reads low
Revolutions per mile
622
was 624 · −1
Switching from 285/55R20 to 305/60R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/55R20 | 305/60R18 | Change |
|---|---|---|---|
| Overall diameter | 32.34″ | 32.41″ | +0.07″ (+0.2%) |
| Sidewall height | 6.17″ (156.8 mm) | 7.20″ (183.0 mm) | +26.3 mm |
| Tread width | 285 mm | 305 mm | +20 mm |
| Circumference | 101.61″ | 101.82″ | +0.21″ |
| Revs per mile | 624 | 622 | −1 |
| Speedometer error | baseline | +0.2% | reads low |
Diameter overlay
285/55R20 305/60R18
Speedometer check
With 305/60R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.1 mph actual
55 mph shown
55.1 mph actual
65 mph shown
65.1 mph actual
75 mph shown
75.2 mph actual
Common questions
- Will 305/60R18 fit instead of 285/55R20?
- Good fit. Within the 3% rule — overall diameter changes only 0.2%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 32.34″ to 32.41″, a change of +0.2%. 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 305/60R18?
- At an indicated 60 mph you would actually be travelling about 60.1 mph — a +0.2% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 285/55R20 and 305/60R18?
- 285/55R20 turns roughly 624 revolutions per mile; 305/60R18 turns roughly 622. That is a change of −1 revolutions per mile, which is what shifts odometer and speedometer readings.