Believe metric conversion on SP is 5 x 127mm
Backspacing is 6.25 inches (so around 160mm), so that would make offset about 51mm on the 17x7.5 and about 58mm on the 16x7.
Give this neat tool a whirl...
http://www.rims-n-tires.com/rt_specs...&text1=&text2=
I hear you about the tyre dudes, they really couldn't give a sh#t, I went in o order 5x copper stt's the other day and they made me feel like I owed them a favour for them doing it. Wheels, well forget it, you mntion anything they can't find in their little books they geta glazed look on their face and look at you s though you just dropped a turd in their lunchbox...easier on the nerves and cheaper to get wheels from the US, at least they try to help....