255/45R20 vs 275/35R20
New overall diameter
27.58″
was 29.04″
True speed at an indicated 60
57.0 mph
speedometer reads high
Revolutions per mile
731
was 695 · +37
Switching from 255/45R20 to 275/35R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 255/45R20 | 275/35R20 | Change |
|---|---|---|---|
| Overall diameter | 29.04″ | 27.58″ | −1.46″ (−5.0%) |
| Sidewall height | 4.52″ (114.8 mm) | 3.79″ (96.3 mm) | −18.5 mm |
| Tread width | 255 mm | 275 mm | +20 mm |
| Circumference | 91.22″ | 86.64″ | −4.58″ |
| Revs per mile | 695 | 731 | +37 |
| Speedometer error | baseline | −5.0% | reads high |
Diameter overlay
255/45R20 275/35R20
Speedometer check
With 275/35R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
23.7 mph actual
35 mph shown
33.2 mph actual
45 mph shown
42.7 mph actual
55 mph shown
52.2 mph actual
65 mph shown
61.7 mph actual
75 mph shown
71.2 mph actual
Common questions
- Will 275/35R20 fit instead of 255/45R20?
- Not recommended. A 5.0% diameter change exceeds the 5% rule. Expect significant speedometer error, potential clearance and rubbing issues, and stress on ABS, stability control and drivetrain components. Your speedometer will read higher than your true speed. Overall diameter goes from 29.04″ to 27.58″, a change of −5.0%. As a rule of thumb, under 3% is safe, 3–5% needs a clearance check, and over 5% is not recommended.
- How far off will my speedometer read on 275/35R20?
- At an indicated 60 mph you would actually be travelling about 57.0 mph — a −5.0% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 255/45R20 and 275/35R20?
- 255/45R20 turns roughly 695 revolutions per mile; 275/35R20 turns roughly 731. That is a change of +37 revolutions per mile, which is what shifts odometer and speedometer readings.