Yes, planning your aero mods is fun, but basic maintenance (new air filter, engine and tranny oil changes, good brakes with calipers that aren't sticking, etc ...) and driving style changes will save you a lot more.
I had a 1996 Civic with slightly wider tires, aftermarket intake and exhaust and for the most part, i drove it like I stole it. I once had 4 sequential tankfuls that each averaged over 45mpg in the middle of summer.
Your goal is easily attainable ... but aero won't have a lot to do with it.