225 is how WIDE the tire is. Stock is 195 or 205, depending on which rims/tires you have from the dealer.
The wider the tire, the more traction.
40 is how "tall" the tire is. 40 is probably good, because it's low profile (which looks good), but the ride should still be ok. If you get 35 or lower, you're gonna have a ROUGH ride and risk popping your tire on potholes.
Get 45 if you want a smooth ride.
And you already know what 18 is for