275/55R20 vs 245/45R20
New overall diameter
28.68″
was 31.91″
True speed at an indicated 60
53.9 mph
speedometer reads high
Revolutions per mile
703
was 632 · +71
Switching from 275/55R20 to 245/45R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 275/55R20 | 245/45R20 | Change |
|---|---|---|---|
| Overall diameter | 31.91″ | 28.68″ | −3.23″ (−10.1%) |
| Sidewall height | 5.95″ (151.3 mm) | 4.34″ (110.3 mm) | −41.0 mm |
| Tread width | 275 mm | 245 mm | −30 mm |
| Circumference | 100.25″ | 90.10″ | −10.14″ |
| Revs per mile | 632 | 703 | +71 |
| Speedometer error | baseline | −10.1% | reads high |
Diameter overlay
275/55R20 245/45R20
Speedometer check
With 245/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
22.5 mph actual
35 mph shown
31.5 mph actual
45 mph shown
40.4 mph actual
55 mph shown
49.4 mph actual
65 mph shown
58.4 mph actual
75 mph shown
67.4 mph actual
Common questions
- Will 245/45R20 fit instead of 275/55R20?
- Not recommended. A 10.1% 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 31.91″ to 28.68″, a change of −10.1%. 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 245/45R20?
- At an indicated 60 mph you would actually be travelling about 53.9 mph — a −10.1% 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 245/45R20?
- 275/55R20 turns roughly 632 revolutions per mile; 245/45R20 turns roughly 703. That is a change of +71 revolutions per mile, which is what shifts odometer and speedometer readings.