275/55R20 vs 255/50R20
New overall diameter
30.04″
was 31.91″
True speed at an indicated 60
56.5 mph
speedometer reads high
Revolutions per mile
671
was 632 · +39
Switching from 275/55R20 to 255/50R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| 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?
- Not recommended. A 5.9% diameter change exceeds the 5% rule. Expect significant speedometer error, potential clearance and rubbing issues, and stress on ABS, stability control and drivetrain components. Your speedometer will read higher than your true speed. Overall diameter goes from 31.91″ to 30.04″, a change of −5.9%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 255/50R20?
- At an indicated 60 mph you would actually be travelling 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 roughly 632 revolutions per mile; 255/50R20 turns roughly 671. That is a change of +39 revolutions per mile, which is what shifts odometer and speedometer readings.