Ok, this is the voice of experience. without dragging this out into a 1000 word essay, here is the condensed version...
225/45-18 this is the size for the factory 18"
205/55-16 this is the size for the factory 16"
195/65-15 this is the size for the factory 15"
notice the second number...45, 55, 65. this number is the profile, or the height of the tire mounted on the wheel. what it mean is percentage of height, relative to the width of the tire. so 225/45 means that the height is no more than 45 percent of 225 millimeter width. same for all the other sizes.
what does this mean in real terms??? it mean that the lower the profile, the shorter the distance between the wheel rim and the bottom of the tire as it rides.
how does that translate into ride quality? less rubber to cushion your ride.
There is one other tire component missing here in our discussion thread....
Sidewall stiffness, also called your speed rating of you tires. Cheap tires = R rating, high performance tires= Z or W rating. And what does that translate into ride quality??? Again, higher speed rating=stiffer sidewall=stiffer ride.
Now, if you're going to be ricky ricer race car driver, then you're probably going to have the Z rated tires so you can hang out with the big dogs, you'll pay in ride quality.
My recommendation is the H rated tire... 130 mph sustained speed. are any of you going to go faster than that? also, stay away from the current flavor of the month "directional" tires. why? if you're running camber adjusted, and with directional tires,you can only go from front to back rotation, not left to right. so if your front and rear camber is different, which race cars are, you'll wheels will shake, and that's perfectly balanced. why? because the fronts and rears wear differently, so when you rotate, your camber is now different and the tires will have to settle into the new camber settings.
Added to this, with higher wall stiffness from Z and W tires, comes more road noise and noise transmission into the cars cabin. But yeah, you get to look cool an show off your stuff.
What do I recommend? This is a great tire... bar none....
Falken Zeix ZE-912
I just bought a set and am throughly amazed, for one reason, they're the quietest tires I've ridden on. I have 205/50-15 in H speed(these are not on my soul) and they handle excellent despite not being a Z or W rated tire. It's the best of 3 worlds for me, ride quality, quiets road noise, and performance.
Falken is made my Sumitomo Tire of Japan. the tire is made in thailand if i'm not mistaken.
I could go into so much more, such as plus sizing for performance/ride, shock replacement for ride/performance and suspension spring types and rate types...
but I think my thousand words is up.
If you'd like more specifics, PM me.
Hope this helps some of you out.