First, add your car to the garage, and check out everybody's garage with the same car, some of them will undoubtedly have good advice for your particular car, and you can see their mods and view fuel logs and compare yours
I'd also set up a fuel log, (I also keep a Fuelly fuel log) to set your base mileage. Whether your car is automatic or stick, there are hundreds of tips on here to help you attain better mileage, and with the fuel log you can track your progress tank to tank. If you're brave enough there are many, many aero mods you can do, even to a fairly aerodynamic car
These guys all seem to swear by the Scangauge since a lot of their cars are compatible. I don't know what it costs since my car's too old to use one, but they talk it up a lot here, you might want to look into buying one
I just posted my own experience with EOC and P&G in a different thread, and I'm still learning the tricks myself, but I have to tell you, you landed in the right site if you're serious about getting everything you can out of your car
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