285/40R21 vs 245/50R20
New overall diameter
29.65″
was 29.98″
True speed at an indicated 60
59.3 mph
your speedo reads high
Revolutions per mile
680
was 673, +8
| Measurement | 285/40R21 | 245/50R20 | Change |
|---|---|---|---|
| Overall diameter | 29.98″ | 29.65″ | −0.33″ (−1.1%) |
| Sidewall height | 4.49″ (114.0 mm) | 4.82″ (122.5 mm) | +8.5 mm |
| Tread width | 285 mm | 245 mm | −40 mm |
| Circumference | 94.17″ | 93.13″ | −1.04″ |
| Revs per mile | 673 | 680 | +8 |
| Speedometer error | baseline | −1.1% | reads high |
Diameter overlay
285/40R21 245/50R20
Speedometer check
With 245/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.7 mph actual
35 mph shown
34.6 mph actual
45 mph shown
44.5 mph actual
55 mph shown
54.4 mph actual
65 mph shown
64.3 mph actual
75 mph shown
74.2 mph actual
Common questions
- Will 245/50R20 fit instead of 285/40R21?
- Overall diameter changes 1.1%; section width changes -40 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional. Overall diameter goes from 29.98″ to 29.65″, a change of −1.1%. 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 245/50R20?
- At an indicated 60 mph you are really doing about 59.3 mph, a −1.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/40R21 and 245/50R20?
- 285/40R21 turns about 673 times per mile and 245/50R20 turns about 680, a difference of +8. Your speedometer and odometer both count rotations, so this number is what throws them off.