275/50R21 vs 265/45R21
New overall diameter
30.39″
was 31.83″
True speed at an indicated 60
57.3 mph
your speedo reads high
Revolutions per mile
664
was 634, +30
| Measurement | 275/50R21 | 265/45R21 | Change |
|---|---|---|---|
| Overall diameter | 31.83″ | 30.39″ | −1.44″ (−4.5%) |
| Sidewall height | 5.41″ (137.5 mm) | 4.69″ (119.3 mm) | −18.3 mm |
| Tread width | 275 mm | 265 mm | −10 mm |
| Circumference | 99.99″ | 95.47″ | −4.51″ |
| Revs per mile | 634 | 664 | +30 |
| Speedometer error | baseline | −4.5% | reads high |
Diameter overlay
275/50R21 265/45R21
Speedometer check
With 265/45R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.9 mph actual
35 mph shown
33.4 mph actual
45 mph shown
43.0 mph actual
55 mph shown
52.5 mph actual
65 mph shown
62.1 mph actual
75 mph shown
71.6 mph actual
Common questions
- Will 265/45R21 fit instead of 275/50R21?
- At 4.5% 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 high, so you are going slower than it says. Overall diameter goes from 31.83″ to 30.39″, a change of −4.5%. 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 265/45R21?
- At an indicated 60 mph you are really doing about 57.3 mph, a −4.5% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/50R21 and 265/45R21?
- 275/50R21 turns about 634 times per mile and 265/45R21 turns about 664, a difference of +30. Your speedometer and odometer both count rotations, so this number is what throws them off.