265/40R21 vs 245/45R20
New overall diameter
28.68″
was 29.35″
True speed at an indicated 60
58.6 mph
your speedo reads high
Revolutions per mile
703
was 687, +16
| Measurement | 265/40R21 | 245/45R20 | Change |
|---|---|---|---|
| Overall diameter | 29.35″ | 28.68″ | −0.67″ (−2.3%) |
| Sidewall height | 4.17″ (106.0 mm) | 4.34″ (110.3 mm) | +4.3 mm |
| Tread width | 265 mm | 245 mm | −20 mm |
| Circumference | 92.19″ | 90.10″ | −2.09″ |
| Revs per mile | 687 | 703 | +16 |
| Speedometer error | baseline | −2.3% | reads high |
Diameter overlay
265/40R21 245/45R20
Speedometer check
With 245/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.4 mph actual
35 mph shown
34.2 mph actual
45 mph shown
44.0 mph actual
55 mph shown
53.8 mph actual
65 mph shown
63.5 mph actual
75 mph shown
73.3 mph actual
Common questions
- Will 245/45R20 fit instead of 265/40R21?
- Overall diameter changes 2.3%; section width changes -20 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 29.35″ to 28.68″, a change of −2.3%. 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 245/45R20?
- At an indicated 60 mph you are really doing about 58.6 mph, a −2.3% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 265/40R21 and 245/45R20?
- 265/40R21 turns about 687 times per mile and 245/45R20 turns about 703, a difference of +16. Your speedometer and odometer both count rotations, so this number is what throws them off.