what is the stock backspacing and offset for the 16" diamond spoke rims? are they different for front and rear like the irocs? if i were to get some wider rims, say 18x9.5", which is 1.5" wider than stock, would i add/subtract 1.5" or .75" to the offset or the backspacing?

 

Offset and backspacing are two different things. Backspace is the distance between the mounting surface of the rim and the backside of the rim. Offset is the distance from the rim's mounting surface to the centerline.

For the factory GTA style (diamond spoke) wheels the rear backspace is 5" and for the front it's 4.25". The offset is Omm for the front and +16mm for the rear.

For more detailed info. on how to fit wider rims go here:
http://www.skulte.com/adapterfaq.html

Hope this helps.