poor cdi fuel economy

[bazzpender]

Just worked out that my c220 cdi only did on average 28mpg on the last tank. During this time i made a couple of long journeys where I was doing 60 mph on cruise control for the journey. i also get a lot of smoke from the car when accelerating.

any ideas on what is causing this poor fuel economy and smoke

Is too much diesel being dumped into the engine?