Very good study report how cars weight effect FE
 

Enjoy ;).

http://aluminumintransportation.org/...th%20cover.pdf

10% weight saving means on average about 3,81 % fuel savings with various petrol engines and car sizes.

10% with various diesel engines and car sizes 3,125 %.

If you optimize the gearing or engine sizes (or use P&G my own interpreration) you can easily more than double those figures

My 2002 Infiniti Q45 uses a great deal of aluminuum (hood, trunk, front fenders etc also engine parts) and other light metals in the design of the car.
It weighs in at 3800 lbs which was less any car in it's class by 500 lbs.
It also came w/ a 2.76 rear ratio.

While I have done some aero mods, Inifiniti gave me a great starting point.
I get 31+ on the road w/ a factory hwy epa of 22mpg

I found that a while ago and added it to the weight reduction wiki.