285/45R21 vs 255/60R19
New overall diameter
31.05″
was 31.10″
True speed at an indicated 60
59.9 mph
your speedo reads high
Revolutions per mile
650
was 649, +1
| Measurement | 285/45R21 | 255/60R19 | Change |
|---|---|---|---|
| Overall diameter | 31.10″ | 31.05″ | −0.05″ (−0.2%) |
| Sidewall height | 5.05″ (128.3 mm) | 6.02″ (153.0 mm) | +24.8 mm |
| Tread width | 285 mm | 255 mm | −30 mm |
| Circumference | 97.70″ | 97.54″ | −0.16″ |
| Revs per mile | 649 | 650 | +1 |
| Speedometer error | baseline | −0.2% | reads high |
Diameter overlay
285/45R21 255/60R19
Speedometer check
With 255/60R19 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.0 mph actual
35 mph shown
34.9 mph actual
45 mph shown
44.9 mph actual
55 mph shown
54.9 mph actual
65 mph shown
64.9 mph actual
75 mph shown
74.9 mph actual
Common questions
- Will 255/60R19 fit instead of 285/45R21?
- 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 high, so you are going slower than it says. Overall diameter goes from 31.10″ to 31.05″, 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 255/60R19?
- At an indicated 60 mph you are really doing about 59.9 mph, a −0.2% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 285/45R21 and 255/60R19?
- 285/45R21 turns about 649 times per mile and 255/60R19 turns about 650, a difference of +1. Your speedometer and odometer both count rotations, so this number is what throws them off.