The figure of 225-250 miles per tank is pretty meaningless as we don't know how much fuel is left when you fill up or do you mean that is on a completely empty/dry tank? Even then we need to know exactly how much you are putting in it. I tend to fill up at around 250 miles and am putting in little over 8-gallons but much better to work out the MPG accurately.
Just fill the tank to the same point each time you fill up (i.e. the gauze in the filler neck) and then make a note of the amount of fuel you put in each time, the first time you do it you will get a figure but if you do it every time you can see how the MPG varies (or not) across different trips, driving styles etc. I do this mentally at the pump every time I fill up (remember 4.54L=1gal, 9L=2gal) but if I want a really accurate figure or I think it has changed significantly I check it with my calculator when I get home. As a rule I get 28MPG when driven hard or 32MPG when I take a bit more care, this is on a completely standard 300TDi 90 with Disco transfer-box and 265/75x16 BFG AT's and calculated on all fuel used over the past 2-years or so. Also the speedo has been checked against a GPS and the (small) difference taken into account when comparing speeds/distances.
