285/65R20 vs 305/55R20
New overall diameter
33.21″
was 34.59″
True speed at an indicated 60
57.6 mph
your speedo reads high
Revolutions per mile
607
was 583, +24
| Measurement | 285/65R20 | 305/55R20 | Change |
|---|---|---|---|
| Overall diameter | 34.59″ | 33.21″ | −1.38″ (−4.0%) |
| Sidewall height | 7.29″ (185.3 mm) | 6.60″ (167.8 mm) | −17.5 mm |
| Tread width | 285 mm | 305 mm | +20 mm |
| Circumference | 108.66″ | 104.33″ | −4.33″ |
| Revs per mile | 583 | 607 | +24 |
| Speedometer error | baseline | −4.0% | reads high |
Diameter overlay
285/65R20 305/55R20
Speedometer check
With 305/55R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.0 mph actual
35 mph shown
33.6 mph actual
45 mph shown
43.2 mph actual
55 mph shown
52.8 mph actual
65 mph shown
62.4 mph actual
75 mph shown
72.0 mph actual
Common questions
- Will 305/55R20 fit instead of 285/65R20?
- At 4.0% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 34.59″ to 33.21″, a change of −4.0%. 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 305/55R20?
- At an indicated 60 mph you are really doing about 57.6 mph, a −4.0% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/65R20 and 305/55R20?
- 285/65R20 turns about 583 times per mile and 305/55R20 turns about 607, a difference of +24. Your speedometer and odometer both count rotations, so this number is what throws them off.