I don’t think your carbon footprint math is right. Every liter of diesel burnt produces about 2.35Kg (a gallon produces about 20lbs) of CO2. This would imply your diesel gets 30km/liter or 70mpg which seems awfully high. Audi’s current, most efficient (US) models are less than half that. You might also care about NOx, SOx, and particulates as well. The diesel is an utter failure as far as particulates go.
It could be an A2 3L, which shares the drivetrain etc. with the VW Lupo 3L, so named because it used just 3L of diesel per 100km (under ideal conditions, of course).
I don't know of any current Audis with similar repeatable fuel economy.
