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If you go with 40 series tires, the overall diameter of the Rim + Tire would be slightly larger than stock, I think about 7-8mm larger. I don't know how this "increase" in size would affect the dynamics of the car's handling, but it's not going to affect your speed or acceleration, at least not noticable to the feel anyway. Omicron is right, the tire would fit just fine.
Oops, I take that back, 245/40/18 is slightly smaller than stock 225/45/18, by about 5mm, according to those tire size calculator. I remember the guy who sold me my Volks told me that generally its better to go slightly larger, correct me if I'm wrong, or he's wrong. I'm running 245/40/18 in the back and 225/45/18 in the front. I'm planning to move up the back to 255/40/18 once the back tires worn out.
Careful when you say smaller. 5mm huh? Is that width or diameter? If diameter, is that actual diameter or circumference? Remember what happened to the mars polar orbiter.
Better is also a relative term. Larger circumference means you travel farther per wheel revolution. That effectively reduces your torque and lowers your reported speed and odo readings. If you rack up lots of commute miles that results in less reported mileage on your car than actual, which helps resale value. However if you autocross then larger circumference reduces your acceleration which hurts performance.