285/40R21 vs 275/45R21
New overall diameter
30.74″
was 29.98″
True speed at an indicated 60
61.5 mph
your speedo reads low
Revolutions per mile
656
was 673, −17
| Measurement | 285/40R21 | 275/45R21 | Change |
|---|---|---|---|
| Overall diameter | 29.98″ | 30.74″ | +0.77″ (+2.6%) |
| Sidewall height | 4.49″ (114.0 mm) | 4.87″ (123.8 mm) | +9.8 mm |
| Tread width | 285 mm | 275 mm | −10 mm |
| Circumference | 94.17″ | 96.59″ | +2.41″ |
| Revs per mile | 673 | 656 | −17 |
| Speedometer error | baseline | +2.6% | reads low |
Diameter overlay
285/40R21 275/45R21
Speedometer check
With 275/45R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.6 mph actual
35 mph shown
35.9 mph actual
45 mph shown
46.2 mph actual
55 mph shown
56.4 mph actual
65 mph shown
66.7 mph actual
75 mph shown
76.9 mph actual
Common questions
- Will 275/45R21 fit instead of 285/40R21?
- Overall diameter changes 2.6%; section width changes -10 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 30.74″, a change of +2.6%. 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 275/45R21?
- At an indicated 60 mph you are really doing about 61.5 mph, a +2.6% 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 275/45R21?
- 285/40R21 turns about 673 times per mile and 275/45R21 turns about 656, a difference of −17. Your speedometer and odometer both count rotations, so this number is what throws them off.