I would guess that the ultimate 123 mileage champ would be a 240d with a 5-speed manual and a turbo. I think there were English market 240d turboes... Maybe with the axle ratio from an 85 300d Otto-matic. And 175 or 185 tires. But it wouldn't be a fun car to drive, or even get very good mileage. My wife's 2003 VW Jetta tdi wagon really never gets less than 35 all around, 45 mpg on the highway.