275/55R20 vs 255/35R20
New overall diameter
27.03″
was 31.91″
True speed at an indicated 60
50.8 mph
your speedo reads high
Revolutions per mile
746
was 632, +114
| Measurement | 275/55R20 | 255/35R20 | Change |
|---|---|---|---|
| Overall diameter | 31.91″ | 27.03″ | −4.88″ (−15.3%) |
| Sidewall height | 5.95″ (151.3 mm) | 3.51″ (89.3 mm) | −62.0 mm |
| Tread width | 275 mm | 255 mm | −20 mm |
| Circumference | 100.25″ | 84.91″ | −15.34″ |
| Revs per mile | 632 | 746 | +114 |
| Speedometer error | baseline | −15.3% | reads high |
Diameter overlay
275/55R20 255/35R20
Speedometer check
With 255/35R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
21.2 mph actual
35 mph shown
29.6 mph actual
45 mph shown
38.1 mph actual
55 mph shown
46.6 mph actual
65 mph shown
55.1 mph actual
75 mph shown
63.5 mph actual
Common questions
- Will 255/35R20 fit instead of 275/55R20?
- 15.3% 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 27.03″, a change of −15.3%. 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 255/35R20?
- At an indicated 60 mph you are really doing about 50.8 mph, a −15.3% 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 255/35R20?
- 275/55R20 turns about 632 times per mile and 255/35R20 turns about 746, a difference of +114. Your speedometer and odometer both count rotations, so this number is what throws them off.