275/55R20 vs 245/40R20
New overall diameter
27.72″
was 31.91″
True speed at an indicated 60
52.1 mph
your speedo reads high
Revolutions per mile
728
was 632, +96
| Measurement | 275/55R20 | 245/40R20 | Change |
|---|---|---|---|
| Overall diameter | 31.91″ | 27.72″ | −4.19″ (−13.1%) |
| Sidewall height | 5.95″ (151.3 mm) | 3.86″ (98.0 mm) | −53.3 mm |
| Tread width | 275 mm | 245 mm | −30 mm |
| Circumference | 100.25″ | 87.07″ | −13.17″ |
| Revs per mile | 632 | 728 | +96 |
| Speedometer error | baseline | −13.1% | reads high |
Diameter overlay
275/55R20 245/40R20
Speedometer check
With 245/40R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
21.7 mph actual
35 mph shown
30.4 mph actual
45 mph shown
39.1 mph actual
55 mph shown
47.8 mph actual
65 mph shown
56.5 mph actual
75 mph shown
65.1 mph actual
Common questions
- Will 245/40R20 fit instead of 275/55R20?
- 13.1% is past the 5% mark, which is where this stops being a tire choice and starts being a project. Expect rubbing, meaningful speedometer error, and extra load on ABS, stability control and the drivetrain. Recalibration is the usual fix. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 31.91″ to 27.72″, a change of −13.1%. 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 245/40R20?
- At an indicated 60 mph you are really doing about 52.1 mph, a −13.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 275/55R20 and 245/40R20?
- 275/55R20 turns about 632 times per mile and 245/40R20 turns about 728, a difference of +96. Your speedometer and odometer both count rotations, so this number is what throws them off.