peugeog/citroen dw8 engin?

hav got a citroen berlingo 2000 reg wit the dw8 engin in it with the semi electronic pump timing . were falin well short of the suposed 600 miles to a tank ive done every consevable thing to save diesel evan fitted low roll resist tires , no rubish in boot tyre presure corect no roofrack compleatly servicd / new filters and the rest doesent hav aircon no turbo . mot emisions well below required. was thinkin iv got 2 older xud engin pugs. where ive set the pump timing fine settin by ear and site they do massive mpg but sound like old taxis , maybe if i adjust the dw8 timing to give more diesel clatter /bit noiser sounding i could improve mpg???. cheers lee . present around 4-450 on a tank..