There is a lot that affect mileage. Tire inflation, condition, size, brand, tread patterns, alignment, fuel and air filters, dragging brakes, length of time driving in one sitting, speed of travel, engine temp, wind conditions, hills, differential gear ratios, AC, fuel quality and type, condition of your injectors, valve clearances.

That said your 200D should be getting more, the 240D's usually get in the upper 20's to low 30's so I'd suspect your 200 should be closer to that range I'd be surprised if you can get it into the 40s. I'd start looking through all the things above to make sure everything is in tip top shape. Start with the basic stuff and work into the more complicated stuff.

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