265/45R21 vs 285/45R21
New overall diameter
31.10″
was 30.39″
True speed at an indicated 60
61.4 mph
speedometer reads low
Revolutions per mile
649
was 664 · −15
Switching from 265/45R21 to 285/45R21? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 265/45R21 | 285/45R21 | Change |
|---|---|---|---|
| Overall diameter | 30.39″ | 31.10″ | +0.71″ (+2.3%) |
| Sidewall height | 4.69″ (119.3 mm) | 5.05″ (128.3 mm) | +9.0 mm |
| Tread width | 265 mm | 285 mm | +20 mm |
| Circumference | 95.47″ | 97.70″ | +2.23″ |
| Revs per mile | 664 | 649 | −15 |
| Speedometer error | baseline | +2.3% | reads low |
Diameter overlay
265/45R21 285/45R21
Speedometer check
With 285/45R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.6 mph actual
35 mph shown
35.8 mph actual
45 mph shown
46.0 mph actual
55 mph shown
56.3 mph actual
65 mph shown
66.5 mph actual
75 mph shown
76.7 mph actual
Common questions
- Will 285/45R21 fit instead of 265/45R21?
- Good fit. Within the 3% rule — overall diameter changes only 2.3%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 30.39″ to 31.10″, a change of +2.3%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 285/45R21?
- At an indicated 60 mph you would actually be travelling about 61.4 mph — a +2.3% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 265/45R21 and 285/45R21?
- 265/45R21 turns roughly 664 revolutions per mile; 285/45R21 turns roughly 649. That is a change of −15 revolutions per mile, which is what shifts odometer and speedometer readings.