255/50R20 vs 275/40R20
New overall diameter
28.66″
was 30.04″
True speed at an indicated 60
57.2 mph
your speedo reads high
Revolutions per mile
704
was 671, +32
| Measurement | 255/50R20 | 275/40R20 | Change |
|---|---|---|---|
| Overall diameter | 30.04″ | 28.66″ | −1.38″ (−4.6%) |
| Sidewall height | 5.02″ (127.5 mm) | 4.33″ (110.0 mm) | −17.5 mm |
| Tread width | 255 mm | 275 mm | +20 mm |
| Circumference | 94.37″ | 90.04″ | −4.33″ |
| Revs per mile | 671 | 704 | +32 |
| Speedometer error | baseline | −4.6% | reads high |
Diameter overlay
255/50R20 275/40R20
Speedometer check
With 275/40R20 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
42.9 mph actual
55 mph shown
52.5 mph actual
65 mph shown
62.0 mph actual
75 mph shown
71.6 mph actual
Common questions
- Will 275/40R20 fit instead of 255/50R20?
- Overall diameter changes 4.6%; 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 30.04″ to 28.66″, a change of −4.6%. 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 275/40R20?
- At an indicated 60 mph you are really doing about 57.2 mph, a −4.6% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 255/50R20 and 275/40R20?
- 255/50R20 turns about 671 times per mile and 275/40R20 turns about 704, a difference of +32. Your speedometer and odometer both count rotations, so this number is what throws them off.