gearbox and aerodynamics same but wheels are larger, but I do more motorway runs so less revving? would a low ratio transfer box be better?

I had this engine in an old F reg range rover ; I've now put it into a slighty newer classic vogue , g reg, which has got 300TDI running gear and high ratio transfer box. Why have i come down from 35mpg to 22mpg? Also if anyone knows of one of these engines for sale, pref low mileage/? Any help...