285/55R20 vs 305/50R20
New overall diameter
32.01″
was 32.34″
True speed at an indicated 60
59.4 mph
your speedo reads high
Revolutions per mile
630
was 624, +7
| Measurement | 285/55R20 | 305/50R20 | Change |
|---|---|---|---|
| Overall diameter | 32.34″ | 32.01″ | −0.33″ (−1.0%) |
| Sidewall height | 6.17″ (156.8 mm) | 6.00″ (152.5 mm) | −4.3 mm |
| Tread width | 285 mm | 305 mm | +20 mm |
| Circumference | 101.61″ | 100.56″ | −1.05″ |
| Revs per mile | 624 | 630 | +7 |
| Speedometer error | baseline | −1.0% | reads high |
Diameter overlay
285/55R20 305/50R20
Speedometer check
With 305/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.7 mph actual
35 mph shown
34.6 mph actual
45 mph shown
44.5 mph actual
55 mph shown
54.4 mph actual
65 mph shown
64.3 mph actual
75 mph shown
74.2 mph actual
Common questions
- Will 305/50R20 fit instead of 285/55R20?
- Diameter moves 1.0%, 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 high, so you are going slower than it says. Overall diameter goes from 32.34″ to 32.01″, a change of −1.0%. 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 305/50R20?
- At an indicated 60 mph you are really doing about 59.4 mph, a −1.0% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/55R20 and 305/50R20?
- 285/55R20 turns about 624 times per mile and 305/50R20 turns about 630, a difference of +7. Your speedometer and odometer both count rotations, so this number is what throws them off.