cant afford the petrol anymore

have s3 truck cab with ford v6 conversion goes great. but its returning less than 10 mpg running very rich and mixture adjustment does alter anything. ques, would you expect less than 10 mpg from this or does anyone think 15-17 would be feasible if set up right???
alternavtively might do a 200tdi swap but i believe the engine mounts will need moving as they were changed from original for the v6.
any thoughts

I think even the Ford explorer gets 15 or so out of the 4 litre V6, thats gotta be heavier than a s3 surely.. LPG?