Okay so I got a 3 1/4" lift from rough country and I'm trying to decide what tires will fit on my stock wheels. I need to know if my tire is a 255/75/17 do I still have to keep the 75 in there or does it matter? I don't know much about tires

 

You'll have to keep the 17.

 

With that lift and some wheel spacers for you factory rims you should be able to run up to 35s.

For 17 inch rims that would be 315/70R17
For 16 inch rims that would be 315/75R16

Just in case you are curious...

Metric tire sizes are composed of 3 numbers, the tire width, the aspect height, and the rim size (in inches oddly enough).

The first number is the tire width in millimeters. So in the above two tires they were 315mm wide, or approximately 12.5 inches wide.
The second number is the aspect height. This is a percentage of the tire's width used to calculate it's height. So for the 315/70R17 that would be 70% of 315mm or 220.5mm. But that only represents half of the tires height, so double it to get 441mm or 17.36 inches.
Finally add the rim size to the above calculated height to get the tires height, so 17 inch rim plus 17.36 inches of rubber equals a 34.36 inch tire.

For comparison the 16 inch rim tire is 34.6 inches tall.

 

Thanks guys. That week I got the rough country lift it gave me the death wobble and I couldn't control my jeep so we snatched it off and put a skyjacker 2.5" on it and I think I'm gonna go with 35's but have to put spacers and I'm gonna paint the stock Rubi rims black