305--the first number--is the section width of the tire. This is how wide the tire is, basically. The 40--the second number--is the aspect ratio, which refers to the sidewall height. So, the 40 means that the sidewall is 40% of 305 (the width). Hope that makes sense. I know it's explained much better on tirerack.com. Also, you said wider tires? If you go any wider than 305, I'm almost positive you are going to have rubbing issues. Trust me, 305 is huge--especially compared to the stock 255 or 265. Don't go wider than 305. It's the perfect fit/look for the truck.