by outside vs. inside i imagine you mean edge to edge vs. mount to mount
if so you have 7.5" wide wheels. wheel width is not edge to edge. my 17x9s measure 10" across. anyway
look through the wheel/tire fitment thread
http://www.zilvia.net/f/showthread.php?t=129865
if you're confused or lazy just use an online calculator
http://www.1010tires.com/WheelOffsetCalculator.asp
find a comparable example...
http://www.zilvia.net/f/showpost.php...7&postcount=85
then compare
7.5 +43mm - 30mm = 13mm effective
vs
8 +5mm front and 9+15mm rear
plug in the numbers and voila
front: 7.5 +13mm has 11mm more outside clearance
rear: 7.5 +13mm has 17mm more outside clearance
of course actual clearance varies depending on tire size and alignment and suspension set up but you get the general idea
tire fitment isn't precise because different brands in the same size don't all sit exactly the same.
anyyyyyyyway imo sell them and get something else
18x7.5 looks bad regardless of how they fit. way too tall to be that skinny