245/40R20 vs 255/35R20
New overall diameter
27.03″
was 27.72″
True speed at an indicated 60
58.5 mph
speedometer reads high
Revolutions per mile
746
was 728 · +19
Switching from 245/40R20 to 255/35R20? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/40R20 | 255/35R20 | Change |
|---|---|---|---|
| Overall diameter | 27.72″ | 27.03″ | −0.69″ (−2.5%) |
| Sidewall height | 3.86″ (98.0 mm) | 3.51″ (89.3 mm) | −8.8 mm |
| Tread width | 245 mm | 255 mm | +10 mm |
| Circumference | 87.07″ | 84.91″ | −2.16″ |
| Revs per mile | 728 | 746 | +19 |
| Speedometer error | baseline | −2.5% | reads high |
Diameter overlay
245/40R20 255/35R20
Speedometer check
With 255/35R20 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.4 mph actual
35 mph shown
34.1 mph actual
45 mph shown
43.9 mph actual
55 mph shown
53.6 mph actual
65 mph shown
63.4 mph actual
75 mph shown
73.1 mph actual
Common questions
- Will 255/35R20 fit instead of 245/40R20?
- Good fit. Within the 3% rule — overall diameter changes only 2.5%. 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 27.72″ to 27.03″, a change of −2.5%. 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 255/35R20?
- At an indicated 60 mph you would actually be travelling about 58.5 mph — a −2.5% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 245/40R20 and 255/35R20?
- 245/40R20 turns roughly 728 revolutions per mile; 255/35R20 turns roughly 746. That is a change of +19 revolutions per mile, which is what shifts odometer and speedometer readings.