295/70R18 vs 305/55R20
New overall diameter
33.21″
was 34.26″
True speed at an indicated 60
58.2 mph
speedometer reads high
Revolutions per mile
607
was 589 · +19
Switching from 295/70R18 to 305/55R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 295/70R18 | 305/55R20 | Change |
|---|---|---|---|
| Overall diameter | 34.26″ | 33.21″ | −1.05″ (−3.1%) |
| Sidewall height | 8.13″ (206.5 mm) | 6.60″ (167.8 mm) | −38.8 mm |
| Tread width | 295 mm | 305 mm | +10 mm |
| Circumference | 107.63″ | 104.33″ | −3.30″ |
| Revs per mile | 589 | 607 | +19 |
| Speedometer error | baseline | −3.1% | reads high |
Diameter overlay
295/70R18 305/55R20
Speedometer check
With 305/55R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.2 mph actual
35 mph shown
33.9 mph actual
45 mph shown
43.6 mph actual
55 mph shown
53.3 mph actual
65 mph shown
63.0 mph actual
75 mph shown
72.7 mph actual
Common questions
- Will 305/55R20 fit instead of 295/70R18?
- Marginal fit — proceed with care. The 3.1% diameter change is between 3% and 5%. It can work, but expect noticeable speedometer error and check wheel-well clearance, scrub radius and gearing before committing. Your speedometer will read higher than your true speed. Overall diameter goes from 34.26″ to 33.21″, a change of −3.1%. 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 305/55R20?
- At an indicated 60 mph you would actually be travelling about 58.2 mph — a −3.1% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 295/70R18 and 305/55R20?
- 295/70R18 turns roughly 589 revolutions per mile; 305/55R20 turns roughly 607. That is a change of +19 revolutions per mile, which is what shifts odometer and speedometer readings.