Well i have a 1983 benz that is my daily driver, it's a 300d turbo diesel, its a brick on wheels... both weight and shape.. however I run it on Straight Vegtable Oil so its eco freindly..ier.
I started just popping in into neutral and idleing (its a auto trans) and coasting to a stop and coasting down hills and I cant say how much more MPG I'm getting but it has made a difference in usage of fuel, I know because I run a 5 gallon veggie tank in the trunk and i can usually go 3 days commuting on that tank before adding 5 more gallons.. well I got a entire extra day out of the 5 gallons..
Its really hard to get a MPG rating as on a veggie conversion I start on diesel till the motor is warm then switch over to veggie so I have two tanks.. can't really do a tank by tank MPG rating, and a tank of diesel last me well over a month, since you only use it to start and shut down the car to flush the veggie out of the system, but I can notice less fuel used in my veggie tank by doing two simple driving habit changes, that bing coasting down hills and coasting to a stop or coasting to slow the car.. not to mention it should save my break pads too
This driving style isn't ideal in a automatic.. but it does work..