245/50R20 vs 285/40R21
New overall diameter
29.98″
was 29.65″
True speed at an indicated 60
60.7 mph
speedometer reads low
Revolutions per mile
673
was 680 · −8
Switching from 245/50R20 to 285/40R21? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/50R20 | 285/40R21 | Change |
|---|---|---|---|
| Overall diameter | 29.65″ | 29.98″ | +0.33″ (+1.1%) |
| Sidewall height | 4.82″ (122.5 mm) | 4.49″ (114.0 mm) | −8.5 mm |
| Tread width | 245 mm | 285 mm | +40 mm |
| Circumference | 93.13″ | 94.17″ | +1.04″ |
| Revs per mile | 680 | 673 | −8 |
| Speedometer error | baseline | +1.1% | reads low |
Diameter overlay
245/50R20 285/40R21
Speedometer check
With 285/40R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.3 mph actual
35 mph shown
35.4 mph actual
45 mph shown
45.5 mph actual
55 mph shown
55.6 mph actual
65 mph shown
65.7 mph actual
75 mph shown
75.8 mph actual
Common questions
- Will 285/40R21 fit instead of 245/50R20?
- Good fit. Within the 3% rule — overall diameter changes only 1.1%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 29.65″ to 29.98″, a change of +1.1%. 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 285/40R21?
- At an indicated 60 mph you would actually be travelling about 60.7 mph — a +1.1% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 245/50R20 and 285/40R21?
- 245/50R20 turns roughly 680 revolutions per mile; 285/40R21 turns roughly 673. That is a change of −8 revolutions per mile, which is what shifts odometer and speedometer readings.