Welcome to ecomodder.
That's a pretty stiff goal you've set.
In the EcoModder garage, there are some other forum members with 40+ MPG Sentras.
EcoModder Fleet list - EcoModder.com
You can click on the cars' name to get some more info on the car, on the MPG to get to the fuel log.
Or click on the user's name to get to their posts (in a roundabout fashion) where you might find more info on how they drive, what mods they made.