275/55R20 vs 255/45R20
New overall diameter
29.04″
was 31.91″
True speed at an indicated 60
54.6 mph
your speedo reads high
Revolutions per mile
695
was 632, +63
| Measurement | 275/55R20 | 255/45R20 | Change |
|---|---|---|---|
| Overall diameter | 31.91″ | 29.04″ | −2.87″ (−9.0%) |
| Sidewall height | 5.95″ (151.3 mm) | 4.52″ (114.8 mm) | −36.5 mm |
| Tread width | 275 mm | 255 mm | −20 mm |
| Circumference | 100.25″ | 91.22″ | −9.03″ |
| Revs per mile | 632 | 695 | +63 |
| Speedometer error | baseline | −9.0% | reads high |
Diameter overlay
275/55R20 255/45R20
Speedometer check
With 255/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
22.7 mph actual
35 mph shown
31.8 mph actual
45 mph shown
40.9 mph actual
55 mph shown
50.0 mph actual
65 mph shown
59.1 mph actual
75 mph shown
68.2 mph actual
Common questions
- Will 255/45R20 fit instead of 275/55R20?
- Overall diameter changes 9.0%; 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 31.91″ to 29.04″, a change of −9.0%. 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 255/45R20?
- At an indicated 60 mph you are really doing about 54.6 mph, a −9.0% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/55R20 and 255/45R20?
- 275/55R20 turns about 632 times per mile and 255/45R20 turns about 695, a difference of +63. Your speedometer and odometer both count rotations, so this number is what throws them off.