235/45R19 vs 235/40R19
New overall diameter
26.40″
was 27.33″
True speed at an indicated 60
58.0 mph
speedometer reads high
Revolutions per mile
764
was 738 · +26
Switching from 235/45R19 to 235/40R19? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 235/45R19 | 235/40R19 | Change |
|---|---|---|---|
| Overall diameter | 27.33″ | 26.40″ | −0.93″ (−3.4%) |
| Sidewall height | 4.16″ (105.8 mm) | 3.70″ (94.0 mm) | −11.8 mm |
| Tread width | 235 mm | 235 mm | +0 mm |
| Circumference | 85.85″ | 82.94″ | −2.91″ |
| Revs per mile | 738 | 764 | +26 |
| Speedometer error | baseline | −3.4% | reads high |
Diameter overlay
235/45R19 235/40R19
Speedometer check
With 235/40R19 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.2 mph actual
35 mph shown
33.8 mph actual
45 mph shown
43.5 mph actual
55 mph shown
53.1 mph actual
65 mph shown
62.8 mph actual
75 mph shown
72.5 mph actual
Common questions
- Will 235/40R19 fit instead of 235/45R19?
- Marginal fit — proceed with care. The 3.4% 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 27.33″ to 26.40″, a change of −3.4%. 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 235/40R19?
- At an indicated 60 mph you would actually be travelling about 58.0 mph — a −3.4% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 235/45R19 and 235/40R19?
- 235/45R19 turns roughly 738 revolutions per mile; 235/40R19 turns roughly 764. That is a change of +26 revolutions per mile, which is what shifts odometer and speedometer readings.