265/50R20 vs 255/45R20
New overall diameter
29.04″
was 30.43″
True speed at an indicated 60
57.2 mph
speedometer reads high
Revolutions per mile
695
was 663 · +32
Switching from 265/50R20 to 255/45R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 265/50R20 | 255/45R20 | Change |
|---|---|---|---|
| Overall diameter | 30.43″ | 29.04″ | −1.40″ (−4.6%) |
| Sidewall height | 5.22″ (132.5 mm) | 4.52″ (114.8 mm) | −17.8 mm |
| Tread width | 265 mm | 255 mm | −10 mm |
| Circumference | 95.61″ | 91.22″ | −4.39″ |
| Revs per mile | 663 | 695 | +32 |
| Speedometer error | baseline | −4.6% | reads high |
Diameter overlay
265/50R20 255/45R20
Speedometer check
With 255/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.9 mph actual
35 mph shown
33.4 mph actual
45 mph shown
42.9 mph actual
55 mph shown
52.5 mph actual
65 mph shown
62.0 mph actual
75 mph shown
71.6 mph actual
Common questions
- Will 255/45R20 fit instead of 265/50R20?
- Marginal fit — proceed with care. The 4.6% diameter change is between 3% and 5%. It can work, but expect noticeable speedometer error and check wheel-well clearance, scrub radius and gearing before committing. Your speedometer will read higher than your true speed. Overall diameter goes from 30.43″ to 29.04″, a change of −4.6%. 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 255/45R20?
- At an indicated 60 mph you would actually be travelling about 57.2 mph — a −4.6% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 265/50R20 and 255/45R20?
- 265/50R20 turns roughly 663 revolutions per mile; 255/45R20 turns roughly 695. That is a change of +32 revolutions per mile, which is what shifts odometer and speedometer readings.