245/50R20 vs 265/40R21
New overall diameter
29.35″
was 29.65″
True speed at an indicated 60
59.4 mph
your speedo reads high
Revolutions per mile
687
was 680, +7
| Measurement | 245/50R20 | 265/40R21 | Change |
|---|---|---|---|
| Overall diameter | 29.65″ | 29.35″ | −0.30″ (−1.0%) |
| Sidewall height | 4.82″ (122.5 mm) | 4.17″ (106.0 mm) | −16.5 mm |
| Tread width | 245 mm | 265 mm | +20 mm |
| Circumference | 93.13″ | 92.19″ | −0.94″ |
| Revs per mile | 680 | 687 | +7 |
| Speedometer error | baseline | −1.0% | reads high |
Diameter overlay
245/50R20 265/40R21
Speedometer check
With 265/40R21 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.7 mph actual
35 mph shown
34.6 mph actual
45 mph shown
44.5 mph actual
55 mph shown
54.4 mph actual
65 mph shown
64.3 mph actual
75 mph shown
74.2 mph actual
Common questions
- Will 265/40R21 fit instead of 245/50R20?
- Overall diameter changes 1.0%; 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 29.65″ to 29.35″, a change of −1.0%. 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 59.4 mph, a −1.0% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 245/50R20 and 265/40R21?
- 245/50R20 turns about 680 times per mile and 265/40R21 turns about 687, a difference of +7. Your speedometer and odometer both count rotations, so this number is what throws them off.