275/55R20 vs 305/50R20
New overall diameter
32.01″
was 31.91″
True speed at an indicated 60
60.2 mph
speedometer reads low
Revolutions per mile
630
was 632 · −2
Switching from 275/55R20 to 305/50R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 275/55R20 | 305/50R20 | Change |
|---|---|---|---|
| Overall diameter | 31.91″ | 32.01″ | +0.10″ (+0.3%) |
| Sidewall height | 5.95″ (151.3 mm) | 6.00″ (152.5 mm) | +1.3 mm |
| Tread width | 275 mm | 305 mm | +30 mm |
| Circumference | 100.25″ | 100.56″ | +0.31″ |
| Revs per mile | 632 | 630 | −2 |
| Speedometer error | baseline | +0.3% | reads low |
Diameter overlay
275/55R20 305/50R20
Speedometer check
With 305/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.1 mph actual
55 mph shown
55.2 mph actual
65 mph shown
65.2 mph actual
75 mph shown
75.2 mph actual
Common questions
- Will 305/50R20 fit instead of 275/55R20?
- Good fit. Within the 3% rule — overall diameter changes only 0.3%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed. Overall diameter goes from 31.91″ to 32.01″, a change of +0.3%. 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 305/50R20?
- At an indicated 60 mph you would actually be travelling about 60.2 mph — a +0.3% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 275/55R20 and 305/50R20?
- 275/55R20 turns roughly 632 revolutions per mile; 305/50R20 turns roughly 630. That is a change of −2 revolutions per mile, which is what shifts odometer and speedometer readings.