235/40R19 vs 225/45R18
New overall diameter
25.97″
was 26.40″
True speed at an indicated 60
59.0 mph
speedometer reads high
Revolutions per mile
777
was 764 · +13
Switching from 235/40R19 to 225/45R18? Everything that changes is below — sidewall, tread width, circumference, revolutions per mile and speedometer error at road speeds.
| Measurement | 235/40R19 | 225/45R18 | Change |
|---|---|---|---|
| Overall diameter | 26.40″ | 25.97″ | −0.43″ (−1.6%) |
| Sidewall height | 3.70″ (94.0 mm) | 3.99″ (101.3 mm) | +7.3 mm |
| Tread width | 235 mm | 225 mm | −10 mm |
| Circumference | 82.94″ | 81.59″ | −1.35″ |
| Revs per mile | 764 | 777 | +13 |
| Speedometer error | baseline | −1.6% | reads high |
Diameter overlay
235/40R19 225/45R18
Speedometer check
With 225/45R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.6 mph actual
35 mph shown
34.4 mph actual
45 mph shown
44.3 mph actual
55 mph shown
54.1 mph actual
65 mph shown
63.9 mph actual
75 mph shown
73.8 mph actual
Common questions
- Will 225/45R18 fit instead of 235/40R19?
- Good fit. Within the 3% rule — overall diameter changes only 1.6%. 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 26.40″ to 25.97″, a change of −1.6%. 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 225/45R18?
- At an indicated 60 mph you would actually be travelling about 59.0 mph — a −1.6% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 235/40R19 and 225/45R18?
- 235/40R19 turns roughly 764 revolutions per mile; 225/45R18 turns roughly 777. That is a change of +13 revolutions per mile, which is what shifts odometer and speedometer readings.