If I were to go with skinnier tires like 225-45-18's when I go to an 18" rim, and that's a big IF, I would then use much better rubber like Michelin Pilot Super Sports to get additional grip and make up for the tire width lost (if I can find the MPSS in that size). The only time I will probably put 225-45-18's on (which is the wrong size btw and does not match the circumference of our 235-40-19) is if I use the 18's for winter tires. I don't care much about handling in the winter or bad weather, and skinnier tires do better in the snow.
EngTech1 is right on the money. Most cars's speedometers read 1-2mph over the actual speed, which puts additional miles to your odometer that you did not drive. The best way to counter that is to go with a slightly taller tire. BTW, I know my speedometer is off by 1mph. There is a police radar display on the side of the road a few miles from me on a 35mph zone. I had the cruise at 40 and the radar read 39. It always reads 1mph less when I drive by it with my '19 Accord. Surprisingly my wife's 2013 Accord always matches the radar display.
Anyway, the 225-45-18 tire is actually still smaller in circumference that our stock 235-40-19, so your speedometer will read 1mph higher than actual speed, which means you are racking up extra miles on the odo and depreciating your car's value faster. The perfect match in 18" is 235-45-18. So don't trust everything that some customer rep at TireRack tells you (GHawkins). Look it up yourself on the many tire size calculators online. Do your own research when spending a large sum of your hard-earned $$, so you don't have to rely on a Rep. that's been on the job for a week or was told to move a certain tire in 225-45-18...
