285/40R21 vs 285/45R21
New overall diameter
31.10″
was 29.98″
True speed at an indicated 60
62.2 mph
your speedo reads low
Revolutions per mile
649
was 673, −24
| Measurement | 285/40R21 | 285/45R21 | Change |
|---|---|---|---|
| Overall diameter | 29.98″ | 31.10″ | +1.12″ (+3.7%) |
| Sidewall height | 4.49″ (114.0 mm) | 5.05″ (128.3 mm) | +14.3 mm |
| Tread width | 285 mm | 285 mm | +0 mm |
| Circumference | 94.17″ | 97.70″ | +3.53″ |
| Revs per mile | 673 | 649 | −24 |
| Speedometer error | baseline | +3.7% | reads low |
Diameter overlay
285/40R21 285/45R21
Speedometer check
With 285/45R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.9 mph actual
35 mph shown
36.3 mph actual
45 mph shown
46.7 mph actual
55 mph shown
57.1 mph actual
65 mph shown
67.4 mph actual
75 mph shown
77.8 mph actual
Common questions
- Will 285/45R21 fit instead of 285/40R21?
- At 3.7% this lands between the 3% and 5% marks. Plenty of people run swaps like this, but you will see real speedometer error, and you want to check the wheel well at full lock and full compression before you buy. Your speedometer will read low, so you are going faster than it says. Overall diameter goes from 29.98″ to 31.10″, a change of +3.7%. 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 285/45R21?
- At an indicated 60 mph you are really doing about 62.2 mph, a +3.7% 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 285/45R21?
- 285/40R21 turns about 673 times per mile and 285/45R21 turns about 649, a difference of −24. Your speedometer and odometer both count rotations, so this number is what throws them off.