285/40R21 vs 285/40R22
New overall diameter
30.98″
was 29.98″
True speed at an indicated 60
62.0 mph
speedometer reads low
Revolutions per mile
651
was 673 · −22
Switching from 285/40R21 to 285/40R22? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 285/40R21 | 285/40R22 | Change |
|---|---|---|---|
| Overall diameter | 29.98″ | 30.98″ | +1.00″ (+3.3%) |
| Sidewall height | 4.49″ (114.0 mm) | 4.49″ (114.0 mm) | +0.0 mm |
| Tread width | 285 mm | 285 mm | +0 mm |
| Circumference | 94.17″ | 97.32″ | +3.14″ |
| Revs per mile | 673 | 651 | −22 |
| Speedometer error | baseline | +3.3% | reads low |
Diameter overlay
285/40R21 285/40R22
Speedometer check
With 285/40R22 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.8 mph actual
35 mph shown
36.2 mph actual
45 mph shown
46.5 mph actual
55 mph shown
56.8 mph actual
65 mph shown
67.2 mph actual
75 mph shown
77.5 mph actual
Common questions
- Will 285/40R22 fit instead of 285/40R21?
- Marginal fit — proceed with care. The 3.3% 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 29.98″ to 30.98″, a change of +3.3%. 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 285/40R22?
- At an indicated 60 mph you would actually be travelling about 62.0 mph — a +3.3% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 285/40R21 and 285/40R22?
- 285/40R21 turns roughly 673 revolutions per mile; 285/40R22 turns roughly 651. That is a change of −22 revolutions per mile, which is what shifts odometer and speedometer readings.