The section of the tyre between the wheel and the road is called the sidewall. If your Kia is fitted with 18-inch wheels it is likely to have 235/45R18 tyres - the 45 refers to the tyre's aspect ratio which is the height of the sidewall expressed as a percentage of the tyre width. 45 profile is relatively narrow.

If you keep your original alloy wheels you will need to fit an 18-inch tyre, but you could change the aspect ratio to 50 or even 55, which would give you a larger sidewall and better ride comfort. Looking on blackcircles.com, you could look at the Toyo Proxes Comfort or Dunlop SP Sport 270 which are good brands at a reasonable price, but there are cheaper options also available.