225/45R17 vs 235/40R18
New overall diameter
25.40″
was 24.97″
True speed at an indicated 60
61.0 mph
your speedo reads low
Revolutions per mile
794
was 808, −14
| Measurement | 225/45R17 | 235/40R18 | Change |
|---|---|---|---|
| Overall diameter | 24.97″ | 25.40″ | +0.43″ (+1.7%) |
| Sidewall height | 3.99″ (101.3 mm) | 3.70″ (94.0 mm) | −7.3 mm |
| Tread width | 225 mm | 235 mm | +10 mm |
| Circumference | 78.45″ | 79.80″ | +1.35″ |
| Revs per mile | 808 | 794 | −14 |
| Speedometer error | baseline | +1.7% | reads low |
Diameter overlay
225/45R17 235/40R18
Speedometer check
With 235/40R18 fitted, when the speedometer shows the indicated speed, your actual speed is:
25 mph shown
25.4 mph actual
35 mph shown
35.6 mph actual
45 mph shown
45.8 mph actual
55 mph shown
55.9 mph actual
65 mph shown
66.1 mph actual
75 mph shown
76.3 mph actual
Common questions
- Will 235/40R18 fit instead of 225/45R17?
- Diameter moves 1.7%, 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 low, so you are going faster than it says. Overall diameter goes from 24.97″ to 25.40″, a change of +1.7%. 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 235/40R18?
- At an indicated 60 mph you are really doing about 61.0 mph, a +1.7% error. The speedometer reads low, so you are going faster than it shows.
- What is the difference in revolutions per mile between 225/45R17 and 235/40R18?
- 225/45R17 turns about 808 times per mile and 235/40R18 turns about 794, a difference of −14. Your speedometer and odometer both count rotations, so this number is what throws them off.