265/45R21 vs 265/40R21
New overall diameter
29.35″
was 30.39″
True speed at an indicated 60
57.9 mph
your speedo reads high
Revolutions per mile
687
was 664, +24
| Measurement | 265/45R21 | 265/40R21 | Change |
|---|---|---|---|
| Overall diameter | 30.39″ | 29.35″ | −1.04″ (−3.4%) |
| Sidewall height | 4.69″ (119.3 mm) | 4.17″ (106.0 mm) | −13.3 mm |
| Tread width | 265 mm | 265 mm | +0 mm |
| Circumference | 95.47″ | 92.19″ | −3.28″ |
| Revs per mile | 664 | 687 | +24 |
| Speedometer error | baseline | −3.4% | reads high |
Diameter overlay
265/45R21 265/40R21
Speedometer check
With 265/40R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.1 mph actual
35 mph shown
33.8 mph actual
45 mph shown
43.5 mph actual
55 mph shown
53.1 mph actual
65 mph shown
62.8 mph actual
75 mph shown
72.4 mph actual
Common questions
- Will 265/40R21 fit instead of 265/45R21?
- At 3.4% 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 30.39″ to 29.35″, a change of −3.4%. 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 265/40R21?
- At an indicated 60 mph you are really doing about 57.9 mph, a −3.4% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 265/45R21 and 265/40R21?
- 265/45R21 turns about 664 times per mile and 265/40R21 turns about 687, a difference of +24. Your speedometer and odometer both count rotations, so this number is what throws them off.