235/40R18 vs 225/40R18
New overall diameter
25.09″
was 25.40″
True speed at an indicated 60
59.3 mph
your speedo reads high
Revolutions per mile
804
was 794, +10
| Measurement | 235/40R18 | 225/40R18 | Change |
|---|---|---|---|
| Overall diameter | 25.40″ | 25.09″ | −0.31″ (−1.2%) |
| Sidewall height | 3.70″ (94.0 mm) | 3.54″ (90.0 mm) | −4.0 mm |
| Tread width | 235 mm | 225 mm | −10 mm |
| Circumference | 79.80″ | 78.81″ | −0.99″ |
| Revs per mile | 794 | 804 | +10 |
| Speedometer error | baseline | −1.2% | reads high |
Diameter overlay
235/40R18 225/40R18
Speedometer check
With 225/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
24.7 mph actual
35 mph shown
34.6 mph actual
45 mph shown
44.4 mph actual
55 mph shown
54.3 mph actual
65 mph shown
64.2 mph actual
75 mph shown
74.1 mph actual
Common questions
- Will 225/40R18 fit instead of 235/40R18?
- Diameter moves 1.2%, comfortably inside the 3% rule. Your speedometer, ABS, traction control and gearing all stay in the range they were calibrated for. Your speedometer will read high, so you are going slower than it says. Overall diameter goes from 25.40″ to 25.09″, a change of −1.2%. The rule of thumb most shops use: under 3% is fine, 3 to 5% needs a clearance check, over 5% needs recalibration.
- How far off will my speedometer read on 225/40R18?
- At an indicated 60 mph you are really doing about 59.3 mph, a −1.2% error. The speedometer reads high, so you are going slower than it shows.
- What is the difference in revolutions per mile between 235/40R18 and 225/40R18?
- 235/40R18 turns about 794 times per mile and 225/40R18 turns about 804, a difference of +10. Your speedometer and odometer both count rotations, so this number is what throws them off.