285/40R22 vs 275/50R21
New overall diameter
31.83″
was 30.98″
True speed at an indicated 60
61.6 mph
your speedo reads low
Revolutions per mile
634
was 651, −17
| Measurement | 285/40R22 | 275/50R21 | Change |
|---|---|---|---|
| Overall diameter | 30.98″ | 31.83″ | +0.85″ (+2.7%) |
| Sidewall height | 4.49″ (114.0 mm) | 5.41″ (137.5 mm) | +23.5 mm |
| Tread width | 285 mm | 275 mm | −10 mm |
| Circumference | 97.32″ | 99.99″ | +2.67″ |
| Revs per mile | 651 | 634 | −17 |
| Speedometer error | baseline | +2.7% | reads low |
Diameter overlay
285/40R22 275/50R21
Speedometer check
With 275/50R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.7 mph actual
35 mph shown
36.0 mph actual
45 mph shown
46.2 mph actual
55 mph shown
56.5 mph actual
65 mph shown
66.8 mph actual
75 mph shown
77.1 mph actual
Common questions
- Will 275/50R21 fit instead of 285/40R22?
- Diameter moves 2.7%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 30.98″ to 31.83″, a change of +2.7%. The rule of thumb most shops use: under 3% is fine, 3 to 5% needs a clearance check, over 5% needs recalibration.
- How far off will my speedometer read on 275/50R21?
- At an indicated 60 mph you are really doing about 61.6 mph, a +2.7% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 285/40R22 and 275/50R21?
- 285/40R22 turns about 651 times per mile and 275/50R21 turns about 634, a difference of −17. Your speedometer and odometer both count rotations, so this number is what throws them off.