Welcome to EM! When you get to 5 posts, post up some pics of your beautiful pony!
"Changing the nut behind the wheel" is the first thing people here always say. If the speed limit is 75 don't do 80, do 75 or better yet 70 (obviously in the slow lane, don't get yourself killed
). Time the lights, so you don't rush up to a red, lot's of other things. Check out the Hypermiling Tips:
100+ Hypermiling Tips.
Those mods you are considering are good. I would do a belly pan (least noticeable, which I would assume you want on your Mustang) with a rear diffuser on the back side (very race-ready). If you do a splitter, I would make sure the grill is blocked as best as possible, otherwise you're moving air for nothing. Depending on how "stock" you want it to look you could use fiberglass, but I would recommend cardboard and/or cloroplast at least for testing purposes. Check out this page as well:
65+ Efficiency Mods
I can see why you got no improvement in MPG for the higher octane fuel. Unless your engine requires it, nothing changes to the FE. Higher octane doesn't mean higher energy content.
Good luck and have fun!
