go from a 205/55r16 to a 235/55r16 tire?

THAT is quite a jump in size. Figure it this way. 205 is the cm of the Width of the tire...to find inches, divide by 25.4 ...205 would then be 8.07 inches wide....and 235 would be 9.25 inches wide... a little over an inch wider.

The 55 in the number, is the percentage of Height to Width. How tall the tire is, from the rim.

So figure 8.07 inches wide, times 55%, is 4.44 inches tall. That is on top, so you also have to figure the bottom of the tire.....so add 4.44 to 4.44 and you get 8.88 inches of rubber. That then is added to the 16 in the numbers. So, 8.88 plus 16, is 24.88 inches tall.

Now, take the 9.25 inches of the 235 tire....times 55%, is 5.09 inches tall, times 2 (top and bottom) makes 10.18 inches....and add that to the 16 inch wheel...you now have a tire that is 26.18 inches tall.

You go from 24.88 to 26.18 inches in tire dimension. And much wider.

It will affect your mileage, your speedometer, and could rub on your fenders....both wide and high.

Check with Tire Rack .com and see if they can tell you what maximum tire size you can use.

Won't fit, the 235 is MUCH taller. Try a 225/50/16 instead.

The first number is the width in millimeters.

The second is the height. It's a percentage of the width. 55% of 235mm is over a full inch taller than 55% of 205mm. If you increase one number, you have to DECREASE the other.

No, you can not use those tires on a Kia Optima. If you could, Kia would have used that huge SUV tire on the Optima from the start.