285/40R21 vs 275/45R20
New overall diameter
29.74″
was 29.98″
True speed at an indicated 60
59.5 mph
speedometer reads high
Revolutions per mile
678
was 673 · +5
Switching from 285/40R21 to 275/45R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/40R21 | 275/45R20 | Change |
|---|---|---|---|
| Overall diameter | 29.98″ | 29.74″ | −0.23″ (−0.8%) |
| Sidewall height | 4.49″ (114.0 mm) | 4.87″ (123.8 mm) | +9.8 mm |
| Tread width | 285 mm | 275 mm | −10 mm |
| Circumference | 94.17″ | 93.44″ | −0.73″ |
| Revs per mile | 673 | 678 | +5 |
| Speedometer error | baseline | −0.8% | reads high |
Diameter overlay
285/40R21 275/45R20
Speedometer check
With 275/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.8 mph actual
35 mph shown
34.7 mph actual
45 mph shown
44.7 mph actual
55 mph shown
54.6 mph actual
65 mph shown
64.5 mph actual
75 mph shown
74.4 mph actual
Common questions
- Will 275/45R20 fit instead of 285/40R21?
- Good fit. Within the 3% rule — overall diameter changes only 0.8%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read higher than your true speed. Overall diameter goes from 29.98″ to 29.74″, a change of −0.8%. 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 59.5 mph — a −0.8% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/40R21 and 275/45R20?
- 285/40R21 turns roughly 673 revolutions per mile; 275/45R20 turns roughly 678. That is a change of +5 revolutions per mile, which is what shifts odometer and speedometer readings.