245/35R20 vs 255/40R19
New overall diameter
27.03″
was 26.75″
True speed at an indicated 60
60.6 mph
speedometer reads low
Revolutions per mile
746
was 754 · −8
Switching from 245/35R20 to 255/40R19? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 245/35R20 | 255/40R19 | Change |
|---|---|---|---|
| Overall diameter | 26.75″ | 27.03″ | +0.28″ (+1.0%) |
| Sidewall height | 3.38″ (85.8 mm) | 4.02″ (102.0 mm) | +16.3 mm |
| Tread width | 245 mm | 255 mm | +10 mm |
| Circumference | 84.04″ | 84.92″ | +0.88″ |
| Revs per mile | 754 | 746 | −8 |
| Speedometer error | baseline | +1.0% | reads low |
Diameter overlay
245/35R20 255/40R19
Speedometer check
With 255/40R19 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.3 mph actual
35 mph shown
35.4 mph actual
45 mph shown
45.5 mph actual
55 mph shown
55.6 mph actual
65 mph shown
65.7 mph actual
75 mph shown
75.8 mph actual
Common questions
- Will 255/40R19 fit instead of 245/35R20?
- Good fit. Within the 3% rule — overall diameter changes only 1.0%. 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 26.75″ to 27.03″, a change of +1.0%. 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 255/40R19?
- At an indicated 60 mph you would actually be travelling about 60.6 mph — a +1.0% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 245/35R20 and 255/40R19?
- 245/35R20 turns roughly 754 revolutions per mile; 255/40R19 turns roughly 746. That is a change of −8 revolutions per mile, which is what shifts odometer and speedometer readings.