245/40R20 vs 255/45R20
New overall diameter
29.04″
was 27.72″
True speed at an indicated 60
62.9 mph
speedometer reads low
Revolutions per mile
695
was 728 · −33
Switching from 245/40R20 to 255/45R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/40R20 | 255/45R20 | Change |
|---|---|---|---|
| Overall diameter | 27.72″ | 29.04″ | +1.32″ (+4.8%) |
| Sidewall height | 3.86″ (98.0 mm) | 4.52″ (114.8 mm) | +16.8 mm |
| Tread width | 245 mm | 255 mm | +10 mm |
| Circumference | 87.07″ | 91.22″ | +4.14″ |
| Revs per mile | 728 | 695 | −33 |
| Speedometer error | baseline | +4.8% | reads low |
Diameter overlay
245/40R20 255/45R20
Speedometer check
With 255/45R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
26.2 mph actual
35 mph shown
36.7 mph actual
45 mph shown
47.1 mph actual
55 mph shown
57.6 mph actual
65 mph shown
68.1 mph actual
75 mph shown
78.6 mph actual
Common questions
- Will 255/45R20 fit instead of 245/40R20?
- Marginal fit — proceed with care. The 4.8% 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 lower than your true speed. Overall diameter goes from 27.72″ to 29.04″, a change of +4.8%. 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 255/45R20?
- At an indicated 60 mph you would actually be travelling about 62.9 mph — a +4.8% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 245/40R20 and 255/45R20?
- 245/40R20 turns roughly 728 revolutions per mile; 255/45R20 turns roughly 695. That is a change of −33 revolutions per mile, which is what shifts odometer and speedometer readings.