305/60R18 vs 305/50R20
New overall diameter
32.01″
was 32.41″
True speed at an indicated 60
59.3 mph
speedometer reads high
Revolutions per mile
630
was 622 · +8
Switching from 305/60R18 to 305/50R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 305/60R18 | 305/50R20 | Change |
|---|---|---|---|
| Overall diameter | 32.41″ | 32.01″ | −0.40″ (−1.2%) |
| Sidewall height | 7.20″ (183.0 mm) | 6.00″ (152.5 mm) | −30.5 mm |
| Tread width | 305 mm | 305 mm | +0 mm |
| Circumference | 101.82″ | 100.56″ | −1.26″ |
| Revs per mile | 622 | 630 | +8 |
| Speedometer error | baseline | −1.2% | reads high |
Diameter overlay
305/60R18 305/50R20
Speedometer check
With 305/50R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.7 mph actual
35 mph shown
34.6 mph actual
45 mph shown
44.4 mph actual
55 mph shown
54.3 mph actual
65 mph shown
64.2 mph actual
75 mph shown
74.1 mph actual
Common questions
- Will 305/50R20 fit instead of 305/60R18?
- Good fit. Within the 3% rule — overall diameter changes only 1.2%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read higher than your true speed. Overall diameter goes from 32.41″ to 32.01″, a change of −1.2%. 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 59.3 mph — a −1.2% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 305/60R18 and 305/50R20?
- 305/60R18 turns roughly 622 revolutions per mile; 305/50R20 turns roughly 630. That is a change of +8 revolutions per mile, which is what shifts odometer and speedometer readings.