Yes thats what i mean by LSD. Quaife and Torsen make them for our trannys, there is a post in the transaxle forum about a good deal on torsen right now. Its best to do now while you are going to have the tranny out of the car anways.
for gas mileage... make sure you have all your proper maitanence done (clean the UIM and LIM, spark plugs, filter, crap like that [UIM = upper intake manifold, LIM= Lower intake manifold] you can find how-to's on the main site, under how-to) Also be sure to shift before your secondaries open (shift before 3.5 to be sure).
hope that helps.
edit: check this thread about svt MPG:
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=1039412&page=0&view=collapsed&sb=5&o=7&fpart=all
note rays post.
