Poor Fuel Economy with my 94 Turbo Diesel

[mk31.9td]

Hey guys, sorry if this isnt the correct forum, I'm new to these forums but I have heard lots of good things about everyones knowledge about diesels so I figured I would try to get some help.
My car is a 1994 Jetta turbo diesel, last summer I had the timing belt done and the fuel pump rebuilt. In January I had the thermostat replaced because the temp gauge wasn't getting up to 90 degrees. Now the problem has come back except now the temp sits between 70 and 90, then it will go up to 90 for a few minutes and go back to between 70 and 90 and sit there. I am just wondering if I got a crap thermostat or what?
So now to the real problem, as of right now I am only getting about 660 km's to a full tank. Last summer I was getting around 750 km's to a full tank and I feel that I should be getting around 800 - 850 km's to a full tank. I know that the thermostat problem must have something to do with it because the oil isn't getting up to temperature. As of right now the car still has the winter tires on it (which I will be getting my summer tires on next week) and the exhaust is broken off at the catalyc converter. I plan to get the fuel and air filters changed next week also. I drive about 20 minutes to and from work everyday so I would really like to get this figured out. So any ideas as to what could be causing the terrible fuel economy would be greatly appreciated.
Car in question:

[Wayland]

Check your IP timing. I went from 47 to 53 mpg on my aaz eco by bumping the timing up from .80 (stock) to 1.05mm

[Vincent Waldon]

And you'll want to sort out your temperature issues before diving in much deeper into mileage issues...one will definitely impact the other.

[burn_your_money]

Removing the roof racks will gain you a few mileage points as well