245/50R20 vs 285/40R21
New overall diameter
29.98″
was 29.65″
True speed at an indicated 60
60.7 mph
your speedo reads low
Revolutions per mile
673
was 680, −8
| 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?
- 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 29.65″ to 29.98″, 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 285/40R21?
- At an indicated 60 mph you are really doing 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 about 680 times per mile and 285/40R21 turns about 673, a difference of −8. Your speedometer and odometer both count rotations, so this number is what throws them off.