Here are some numbers from my maiden voyage with my Dehler DJ (high compression, premium fuel)  factory syncro 16.  While I have a de-coupler, I have removed the front drive-shaft due to some vibrations:

    Statistically speaking, about 2,500 kms (1,550 miles).  Fuel economy varied greatly with speed, 19 mpg (at 110 kph (68 mph)), then 22 mpg then 24 mpg to 30 mpg as I slowed to 60 kph (under 40 mph) on some back roads.  On average (6 tanks of gas) the fuel economy was 12.45 L/100 km, or 8 km/l, or 22.687 mpg.

Keith Ovregaard <kovregaard@gmail.com> wrote:                             Be interesting to hear from others who have tracked the fuel efficiency differences with and w/o the decoupler.    Anyone?  Keith O