265/40R21 vs 275/45R21
New overall diameter
30.74″
was 29.35″
True speed at an indicated 60
62.9 mph
speedometer reads low
Revolutions per mile
656
was 687 · −31
Switching from 265/40R21 to 275/45R21? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 265/40R21 | 275/45R21 | Change |
|---|---|---|---|
| Overall diameter | 29.35″ | 30.74″ | +1.40″ (+4.8%) |
| Sidewall height | 4.17″ (106.0 mm) | 4.87″ (123.8 mm) | +17.8 mm |
| Tread width | 265 mm | 275 mm | +10 mm |
| Circumference | 92.19″ | 96.59″ | +4.39″ |
| Revs per mile | 687 | 656 | −31 |
| Speedometer error | baseline | +4.8% | reads low |
Diameter overlay
265/40R21 275/45R21
Speedometer check
With 275/45R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.2 mph actual
35 mph shown
36.7 mph actual
45 mph shown
47.1 mph actual
55 mph shown
57.6 mph actual
65 mph shown
68.1 mph actual
75 mph shown
78.6 mph actual
Common questions
- Will 275/45R21 fit instead of 265/40R21?
- Marginal fit — proceed with care. The 4.8% 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 lower than your true speed. Overall diameter goes from 29.35″ to 30.74″, a change of +4.8%. 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 275/45R21?
- At an indicated 60 mph you would actually be travelling about 62.9 mph — a +4.8% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 265/40R21 and 275/45R21?
- 265/40R21 turns roughly 687 revolutions per mile; 275/45R21 turns roughly 656. That is a change of −31 revolutions per mile, which is what shifts odometer and speedometer readings.