275/45R20 vs 275/40R20
New overall diameter
28.66″
was 29.74″
True speed at an indicated 60
57.8 mph
speedometer reads high
Revolutions per mile
704
was 678 · +26
Switching from 275/45R20 to 275/40R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 275/45R20 | 275/40R20 | Change |
|---|---|---|---|
| Overall diameter | 29.74″ | 28.66″ | −1.08″ (−3.6%) |
| Sidewall height | 4.87″ (123.8 mm) | 4.33″ (110.0 mm) | −13.8 mm |
| Tread width | 275 mm | 275 mm | +0 mm |
| Circumference | 93.44″ | 90.04″ | −3.40″ |
| Revs per mile | 678 | 704 | +26 |
| Speedometer error | baseline | −3.6% | reads high |
Diameter overlay
275/45R20 275/40R20
Speedometer check
With 275/40R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.1 mph actual
35 mph shown
33.7 mph actual
45 mph shown
43.4 mph actual
55 mph shown
53.0 mph actual
65 mph shown
62.6 mph actual
75 mph shown
72.3 mph actual
Common questions
- Will 275/40R20 fit instead of 275/45R20?
- 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 higher than your true speed. Overall diameter goes from 29.74″ to 28.66″, 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 275/40R20?
- At an indicated 60 mph you would actually be travelling about 57.8 mph — a −3.6% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/45R20 and 275/40R20?
- 275/45R20 turns roughly 678 revolutions per mile; 275/40R20 turns roughly 704. That is a change of +26 revolutions per mile, which is what shifts odometer and speedometer readings.