285/45R22 vs 285/45R21
New overall diameter
31.10″
was 32.10″
True speed at an indicated 60
58.1 mph
speedometer reads high
Revolutions per mile
649
was 628 · +20
Switching from 285/45R22 to 285/45R21? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/45R22 | 285/45R21 | Change |
|---|---|---|---|
| Overall diameter | 32.10″ | 31.10″ | −1.00″ (−3.1%) |
| Sidewall height | 5.05″ (128.3 mm) | 5.05″ (128.3 mm) | +0.0 mm |
| Tread width | 285 mm | 285 mm | +0 mm |
| Circumference | 100.84″ | 97.70″ | −3.14″ |
| Revs per mile | 628 | 649 | +20 |
| Speedometer error | baseline | −3.1% | reads high |
Diameter overlay
285/45R22 285/45R21
Speedometer check
With 285/45R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.2 mph actual
35 mph shown
33.9 mph actual
45 mph shown
43.6 mph actual
55 mph shown
53.3 mph actual
65 mph shown
63.0 mph actual
75 mph shown
72.7 mph actual
Common questions
- Will 285/45R21 fit instead of 285/45R22?
- Marginal fit — proceed with care. The 3.1% 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 32.10″ to 31.10″, a change of −3.1%. 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 285/45R21?
- At an indicated 60 mph you would actually be travelling about 58.1 mph — a −3.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/45R22 and 285/45R21?
- 285/45R22 turns roughly 628 revolutions per mile; 285/45R21 turns roughly 649. That is a change of +20 revolutions per mile, which is what shifts odometer and speedometer readings.