I would opt out on lowering springs. If you suspension needs to be replaced clean it up with OEM parts. Don't look at just springs or oil on the struct/shock. Old parts are just old parts. 100K on a shocks, replace them. Springs seem to sag or have alot of dip under braking, replace them. Go after your suspension bushings and steering components. Depending on driving conditions, a better handling car will allow you to maintain higher speeds and eliminate the brake/gas through turns, corners, ramps, etc.
From someone who has been working with a non-eco minded vehicle let be share of of my best/worst experiences, Maybe it will help you with some guiding light!
Best:
-Scan Gauge/Ultra Gauge (I recommend UG because I have one and it is ~$50-$70 cheaper
)
-Front bumper air blocks and air barriers.
-Wheel covers/moon discs
-Tire Combo
ONLY if you need tires, else you're wasting money.
-Basic maintenance plugs, filters, synthetic fluids. Check your brakes, clean up the hardware, dragging brakes will steal alot from a smaller metro engine.
-Driver habits, route planning, traffic avoidance. Case in point, normal commute with morning traffic is 25.x right now with my Subaru, I worked Christmas eve day and managed 28.2 MPG with the lack of traffic.
-Weight Reduction, If you're drive includes alot of stop/go and points where you need to accelerate weight is not your friend. If you drive alot of continuous highway, weight is more negotiable.
Worst
-Vacuum Gauge Why: Cheap Sunpro $20-30, vacuum tees and hose to splice $5, Gauge pod or in my cause custom pod ~15-30, total $45-60 bucks. UG was $80 shipped and doubles as a $50 code reader, and provide all the data that you need!
-Wheel Skirts Why: Flame if you want, but building these are time consuming, potentially dangerous, and build costs could be high if you want to paint match, quality material, etc. Smooth wheel covers and see where you're at!
-LED Lights, swapping to HID etc. Unless you plan on going alternator-less, there are minimal gains to be had and alot of money to spend.
Hopefully that helps!