285/70R18 vs 305/55R20
New overall diameter
33.21″
was 33.71″
True speed at an indicated 60
59.1 mph
your speedo reads high
Revolutions per mile
607
was 598, +9
| Measurement | 285/70R18 | 305/55R20 | Change |
|---|---|---|---|
| Overall diameter | 33.71″ | 33.21″ | −0.50″ (−1.5%) |
| Sidewall height | 7.85″ (199.5 mm) | 6.60″ (167.8 mm) | −31.8 mm |
| Tread width | 285 mm | 305 mm | +20 mm |
| Circumference | 105.90″ | 104.33″ | −1.57″ |
| Revs per mile | 598 | 607 | +9 |
| Speedometer error | baseline | −1.5% | reads high |
Diameter overlay
285/70R18 305/55R20
Speedometer check
With 305/55R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.6 mph actual
35 mph shown
34.5 mph actual
45 mph shown
44.3 mph actual
55 mph shown
54.2 mph actual
65 mph shown
64.0 mph actual
75 mph shown
73.9 mph actual
Common questions
- Will 305/55R20 fit instead of 285/70R18?
- Overall diameter changes 1.5%; section width changes 20 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional. Overall diameter goes from 33.71″ to 33.21″, a change of −1.5%. 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 59.1 mph, a −1.5% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/70R18 and 305/55R20?
- 285/70R18 turns about 598 times per mile and 305/55R20 turns about 607, a difference of +9. Your speedometer and odometer both count rotations, so this number is what throws them off.