Question about backspacing

[hellraiser_88]

I plan on puting a new set of wheels on my truck, I plan to get a set of Coys C-5 17x7 (see bottom for specs). My truck is lowered 3" in the front and 4" in the back. My question(s) are;
1. What is backspacing?
2. Do I need it?
3. How much do I need if I need backspacing?

Thanks --nate
http://img.photobucket.com/albums/v668/gmctrucks/Wheels/c5_polished.jpg

[20Xtreme02]

Backspacing refers to the amount of the wheel inside of the spokes or mounting flange where the wheel is bolted to the hub. Front wheel drive cars have lots of backspacing or offset. The spokes are mounted on or near the outside edge of the wheel. Rear wheel drive cars and truck have less backspacing putting the spokes of the wheel near the center of the rim. You should look for wheels that have the offset or backspacing near what the stock wheels have. If you go with more backspacing the wheels and tires will get moved further inside the wheelwells, possibly causing the tires to rub, especially on the front when turning. If you go with less offset, the wheels and tires will get moved outside the wheelwells which is illegal (if you care about that sort of thing). Another issue with wheels outside the wells is that your tires will throw dirt and rocks on your paint. A good wheel and tire shop will help you get wheels of the right offset and size.

[Sparky2263]

If you lay the wheel on a flat surface (no tire mounted) and measure from the surface to the flange of the wheel (part that goes against the disc or drum) that's your backspacing.

To see how much you can use, you need 2 straight edges. 1 goes against the drum/disc mounting surface, the other goes inside the wheel well (imagine the closest the tire can get inside). You have to account for tire bulge (1-1 1/2") on the rear and turning radius on the front.