275/45R21 vs 285/40R21
New overall diameter
29.98″
was 30.74″
True speed at an indicated 60
58.5 mph
speedometer reads high
Revolutions per mile
673
was 656 · +17
Switching from 275/45R21 to 285/40R21? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 275/45R21 | 285/40R21 | Change |
|---|---|---|---|
| Overall diameter | 30.74″ | 29.98″ | −0.77″ (−2.5%) |
| Sidewall height | 4.87″ (123.8 mm) | 4.49″ (114.0 mm) | −9.8 mm |
| Tread width | 275 mm | 285 mm | +10 mm |
| Circumference | 96.59″ | 94.17″ | −2.41″ |
| Revs per mile | 656 | 673 | +17 |
| Speedometer error | baseline | −2.5% | reads high |
Diameter overlay
275/45R21 285/40R21
Speedometer check
With 285/40R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.4 mph actual
35 mph shown
34.1 mph actual
45 mph shown
43.9 mph actual
55 mph shown
53.6 mph actual
65 mph shown
63.4 mph actual
75 mph shown
73.1 mph actual
Common questions
- Will 285/40R21 fit instead of 275/45R21?
- Good fit. Within the 3% rule — overall diameter changes only 2.5%. 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 30.74″ to 29.98″, a change of −2.5%. 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 285/40R21?
- At an indicated 60 mph you would actually be travelling about 58.5 mph — a −2.5% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/45R21 and 285/40R21?
- 275/45R21 turns roughly 656 revolutions per mile; 285/40R21 turns roughly 673. That is a change of +17 revolutions per mile, which is what shifts odometer and speedometer readings.