Ok ummmm I like the look of the 255/40's in the back with Espelirs springs, fills up a nice gap, I would like to go with 245'/40's but afraid it won't fill up the fender gap as much. I know the 255 is the width but the second number is the percentage of the height as compared to width right? basically I am sking if a 245 will have the same "sidewall" height as a 255????????

 

No they will not have the same sidewall height. The 255 will have a larger overall diameter than the 245 assuming the same sidewall percentage. Here are the specs:

245/40/17 overall diameter: 24.71"
255/40/17 overall diameter: 25.03"

 

Even without doing the math there's no way that
255 x .4 = 245 x .4

 

Depending on the offset of the wheels and the width the 255/40R17 will still work and fill up more of the fender.
If I can help with the tires let me know.