15 mpg? A well tuned B20 should be getting much better than that.

Environmental and political reasons aside, there is very little economical arguement for swapping a diesel into an existing car unless you get the donor car very cheaply. Besides the aforementioned RWD conversion issues, the TDI is also a computer controlled engine to the Nth degree. Amoung other things it uses a drive-by-wire accelerator pedal. The only easy way to do a conversion is to buy a wrecked but running TDI and swap out the entire engine and associated ECU wiring.

Anything is possible. Just takes time and money.

I open the hood of my TDI and get chills thinking about stuffing all of that into the tight confines of a 1800 engine bay. The problems/issues to solve boggle the mind... It could be a really neat conversion, though... especially since a typical 1800 is about 500 lbs lighter than a Mk IV chassis and a TDI can be tuned to produce enough torque melt the rear tires on an 1800 into little black puddles under the car. >:)
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'73 1800 ES 'project', g/f's '73 1800 ES