275/55R20 vs 255/50R20
New overall diameter
30.04″
was 31.91″
True speed at an indicated 60
56.5 mph
your speedo reads high
Revolutions per mile
671
was 632, +39
| Measurement | 275/55R20 | 255/50R20 | Change |
|---|---|---|---|
| Overall diameter | 31.91″ | 30.04″ | −1.87″ (−5.9%) |
| Sidewall height | 5.95″ (151.3 mm) | 5.02″ (127.5 mm) | −23.8 mm |
| Tread width | 275 mm | 255 mm | −20 mm |
| Circumference | 100.25″ | 94.37″ | −5.88″ |
| Revs per mile | 632 | 671 | +39 |
| Speedometer error | baseline | −5.9% | reads high |
Diameter overlay
275/55R20 255/50R20
Speedometer check
With 255/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.5 mph actual
35 mph shown
32.9 mph actual
45 mph shown
42.4 mph actual
55 mph shown
51.8 mph actual
65 mph shown
61.2 mph actual
75 mph shown
70.6 mph actual
Common questions
- Will 255/50R20 fit instead of 275/55R20?
- 5.9% is past the 5% mark, which is where this stops being a tire choice and starts being a project. Expect rubbing, meaningful speedometer error, and extra load on ABS, stability control and the drivetrain. Recalibration is the usual fix. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 31.91″ to 30.04″, a change of −5.9%. 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 255/50R20?
- At an indicated 60 mph you are really doing about 56.5 mph, a −5.9% 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 255/50R20?
- 275/55R20 turns about 632 times per mile and 255/50R20 turns about 671, a difference of +39. Your speedometer and odometer both count rotations, so this number is what throws them off.