size matters! Guys, the only real numbers that significantly affect your ride are the first (width, as in 255) and second, sidewall height (as in 55) which is called the aspect
ratio So a 255/65/18 has a
softer ride than a 255/55/18 because the sidewall height is the
ratio of :
a) 255 x 65% = 165.8 mm of sidewall height (more sidewall= softer)
b) 255 x 55% = 140.3 mm of sidewall height (less sidewall = stiffer)
So a) is a taller sidewall by 25.5 mm. (about 1 inch!) That gives you a softer ride. The 18, 19, or even a 20" is irrelevant to ride quality concerns, as it is just the rubber tires
inner diameter and can have almost any mix of the first 2 numbers. Of course "low profile" tires: 19",20", 22" usually do have
smaller sidewalls and thus give incredible handling characterisitics, at price of ride. Your call.
Often, same size tire is available, but in different rim sizes, like 18 or 19" . They will ride about the same, if first 2 numbers are same, but the larger rim size just yields a larger, outside diameter tire, with a tiny bit better ride, just by sheer larger overall tire size. (if first 2 numbers are same, when rim changes to larger size, the only way to do it is to increase overall tires size , in order to maintain that same aspect ratio height) Not all vehicles will have clearance to fit a larger rim with same rubber size (first 2 numbers). Check first.
My SRX has 235/55/20" optional tires/rims. The base tires/rims were 235/65/18". Since
55% of 235mm is
less than
65% of 235mm, my sidewalls are shorter and ride stiffer, than if I took the base tires/rim package. The overall, freestanding tire height is the same! (wheel well won't take any/much larger without rubbing) Difference is in the shorter
sidewall height, to accomadate my larger 20" rim, thus stiffer ride quality.
To be totally correct, yes, as stated above by TSDUKE, rim/tire weight has some effect on ride, usually nowhere as critiical as sidewall size, in my experience however.
Here is a great chart , with pix and plug-ins, for deciding if a tire comparison will fit your overall, wheel-well opening, in different aspect/diameter size tires.
http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp
TEST DRIVE the car, with both sizes, if the option is offered. The ride and cost of replacement tire will change, often a lot!
Hope that helps.
MM
PS....yes I am a ride quality loon! LOL