285/40R21 vs 255/50R20
New overall diameter
30.04″
was 29.98″
True speed at an indicated 60
60.1 mph
your speedo reads low
Revolutions per mile
671
was 673, −1
| Measurement | 285/40R21 | 255/50R20 | Change |
|---|---|---|---|
| Overall diameter | 29.98″ | 30.04″ | +0.06″ (+0.2%) |
| Sidewall height | 4.49″ (114.0 mm) | 5.02″ (127.5 mm) | +13.5 mm |
| Tread width | 285 mm | 255 mm | −30 mm |
| Circumference | 94.17″ | 94.37″ | +0.20″ |
| Revs per mile | 673 | 671 | −1 |
| Speedometer error | baseline | +0.2% | reads low |
Diameter overlay
285/40R21 255/50R20
Speedometer check
With 255/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.1 mph actual
55 mph shown
55.1 mph actual
65 mph shown
65.1 mph actual
75 mph shown
75.2 mph actual
Common questions
- Will 255/50R20 fit instead of 285/40R21?
- Diameter moves 0.2%, 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 29.98″ to 30.04″, a change of +0.2%. 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 255/50R20?
- At an indicated 60 mph you are really doing about 60.1 mph, a +0.2% 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 255/50R20?
- 285/40R21 turns about 673 times per mile and 255/50R20 turns about 671, a difference of −1. Your speedometer and odometer both count rotations, so this number is what throws them off.