Grand Cherokee CRD - naff fuel consumption!

[shortshift]

I've recently bought an 05 Grand Cherokee CRD (I think it's a Merc engine). Pulls like and train and great to drive but can't get more than 21 mpg!? It's often as low as 17 mpg, even after a long run. It's supposed to have an average of 27.7 mpg. It had a torque converter problem which was sorted by changing the transmission fluid. I was hoping that was causing the poor consumption but it hasn't changed. Anyone got any ideas?

[shortshift]

Thanks for all the good advice. Calculated actual MPG based on a full tank using plug in sat nav to check distance and got 28.8 over a 3 day mixed run. Much happier The dash read out is showing 19.6 mpg! Guess it's either faulty or screwed!