MPG Enhancements

[Volvoist]

With the rising (yet again) cost of fuel, I figured I would ask the age old question of how to get better MPG. I have worked on TDI's and on older 1.6 diesels, and it is said that adjusting the timing (advancing it slightly) helps with fuel economy. Is the same to be said on the D24/D24T engine? I am more focused on doing this with a D24 engine. European spec vehicles (just about any car made) got, or gets better MPG than their American spec counterparts, which is rather annoying. Did the D24/D24T get better MPG in Europe? If so, what made this happen? In all of my D24/D24T engines, my averages were as following:

1980 200 M46 series all averaged between 38-41 MPG on the highway depending on how I drove and the temperature, and got the best mileage out of any other year Volvo diesel I owned. I owned 3 of these, and the result was the same. Not quite sure why. Perhaps it was the different cylinder head design and the different timing spec for the 1980 model year.
1982-1984 200 M46 all averaged around 35-36 MPG on the highway depending on how I drove and the ambient temperature.
1984 200 BW automatic all averaged 30 MPG on the highway depending on how I drove and the ambient temperature.
1985-1986 700 ZF22 all averaged 30 MPG on the highway depending on how I drove and the ambient temperature.

These were my findings over the past 10 years or so, and after owning many D24/D24T equipped Volvos. Has anyone achieved better results with simple modifications such as advancing timing slightly or injector modification?