275/50R21 vs 265/45R21
New overall diameter
30.39″
was 31.83″
True speed at an indicated 60
57.3 mph
speedometer reads high
Revolutions per mile
664
was 634 · +30
Switching from 275/50R21 to 265/45R21? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 275/50R21 | 265/45R21 | Change |
|---|---|---|---|
| Overall diameter | 31.83″ | 30.39″ | −1.44″ (−4.5%) |
| Sidewall height | 5.41″ (137.5 mm) | 4.69″ (119.3 mm) | −18.3 mm |
| Tread width | 275 mm | 265 mm | −10 mm |
| Circumference | 99.99″ | 95.47″ | −4.51″ |
| Revs per mile | 634 | 664 | +30 |
| Speedometer error | baseline | −4.5% | reads high |
Diameter overlay
275/50R21 265/45R21
Speedometer check
With 265/45R21 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
43.0 mph actual
55 mph shown
52.5 mph actual
65 mph shown
62.1 mph actual
75 mph shown
71.6 mph actual
Common questions
- Will 265/45R21 fit instead of 275/50R21?
- Marginal fit — proceed with care. The 4.5% 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 31.83″ to 30.39″, a change of −4.5%. 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 265/45R21?
- At an indicated 60 mph you would actually be travelling about 57.3 mph — a −4.5% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/50R21 and 265/45R21?
- 275/50R21 turns roughly 634 revolutions per mile; 265/45R21 turns roughly 664. That is a change of +30 revolutions per mile, which is what shifts odometer and speedometer readings.