275/40R20 vs 275/45R20
New overall diameter
29.74″
was 28.66″
True speed at an indicated 60
62.3 mph
speedometer reads low
Revolutions per mile
678
was 704 · −26
Switching from 275/40R20 to 275/45R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 275/40R20 | 275/45R20 | Change |
|---|---|---|---|
| Overall diameter | 28.66″ | 29.74″ | +1.08″ (+3.8%) |
| Sidewall height | 4.33″ (110.0 mm) | 4.87″ (123.8 mm) | +13.8 mm |
| Tread width | 275 mm | 275 mm | +0 mm |
| Circumference | 90.04″ | 93.44″ | +3.40″ |
| Revs per mile | 704 | 678 | −26 |
| Speedometer error | baseline | +3.8% | reads low |
Diameter overlay
275/40R20 275/45R20
Speedometer check
With 275/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.9 mph actual
35 mph shown
36.3 mph actual
45 mph shown
46.7 mph actual
55 mph shown
57.1 mph actual
65 mph shown
67.5 mph actual
75 mph shown
77.8 mph actual
Common questions
- Will 275/45R20 fit instead of 275/40R20?
- Marginal fit — proceed with care. The 3.8% 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 28.66″ to 29.74″, a change of +3.8%. 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 275/45R20?
- At an indicated 60 mph you would actually be travelling about 62.3 mph — a +3.8% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 275/40R20 and 275/45R20?
- 275/40R20 turns roughly 704 revolutions per mile; 275/45R20 turns roughly 678. That is a change of −26 revolutions per mile, which is what shifts odometer and speedometer readings.