Will this tire fit?
Compare your stock tire size with a new one — diameter, width, speedometer error and rim width, side by side.
Yes — 225/40R18 is within 3% of 245/40R18
| Spec | 245/40R18 | 225/40R18 | Difference |
|---|---|---|---|
| Overall diameter (in) | 25.72 | 25.09 | -0.6" (-2.45%) |
| Section width (mm) | 245 | 225 | -20 mm |
| Sidewall (mm) | 98 | 90 | -8 mm |
| Circumference (in) | 80.79 | 78.81 | -2" |
| Revs per mile | 784 | 804 | +20 |
Speedometer error
Your speedometer is calibrated for 245/40R18. With 225/40R18, your real speed is:
| Speedometer shows | Actual speed |
|---|---|
| 30 mph | 29.3 mph |
| 45 mph | 43.9 mph |
| 60 mph | 58.5 mph |
| 75 mph | 73.2 mph |
Clearance
Ride height changes by -8 mm.
Tread edge moves -10 mm on each side (narrower).
Rim width for 225/40R18: about 6.5–8.5".
Frequently asked questions
How much can a tire’s diameter differ from stock?
The widely used rule of thumb is to stay within 3% of the original overall diameter. Beyond that, speedometer error grows, gearing and ABS calibration are affected, and the tire is more likely to rub.
How is overall tire diameter calculated?
Overall diameter = rim diameter + 2 × sidewall height. Sidewall height = section width × aspect ratio ÷ 100, converted from millimetres to inches (÷ 25.4). For 225/45R17 that is 17 + 2 × (225 × 0.45 ÷ 25.4) ≈ 24.97 inches.
Will a bigger tire make my speedometer read low?
Yes. A taller tire travels further per revolution, so at an indicated 60 mph you are actually going 60 × (new diameter ÷ old diameter).
Can I put a wider tire on my stock rims?
Only if the rim width is inside the approved range for that tire. The calculator shows an approximate range; the tire maker’s data sheet has the exact figures.