265/40R21 vs 265/45R21
New overall diameter
30.39″
was 29.35″
True speed at an indicated 60
62.1 mph
speedometer reads low
Revolutions per mile
664
was 687 · −24
Switching from 265/40R21 to 265/45R21? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 265/40R21 | 265/45R21 | Change |
|---|---|---|---|
| Overall diameter | 29.35″ | 30.39″ | +1.04″ (+3.6%) |
| Sidewall height | 4.17″ (106.0 mm) | 4.69″ (119.3 mm) | +13.3 mm |
| Tread width | 265 mm | 265 mm | +0 mm |
| Circumference | 92.19″ | 95.47″ | +3.28″ |
| Revs per mile | 687 | 664 | −24 |
| Speedometer error | baseline | +3.6% | reads low |
Diameter overlay
265/40R21 265/45R21
Speedometer check
With 265/45R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.9 mph actual
35 mph shown
36.2 mph actual
45 mph shown
46.6 mph actual
55 mph shown
57.0 mph actual
65 mph shown
67.3 mph actual
75 mph shown
77.7 mph actual
Common questions
- Will 265/45R21 fit instead of 265/40R21?
- Marginal fit — proceed with care. The 3.6% 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 lower than your true speed. Overall diameter goes from 29.35″ to 30.39″, a change of +3.6%. 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 265/45R21?
- At an indicated 60 mph you would actually be travelling about 62.1 mph — a +3.6% 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 265/45R21?
- 265/40R21 turns roughly 687 revolutions per mile; 265/45R21 turns roughly 664. That is a change of −24 revolutions per mile, which is what shifts odometer and speedometer readings.