Hi Dan,Just had my 2012 2.0 134 TDI mapped now 185 by Zensport Essex.
I don't drive mine slow but not mad and cover over 1000 miles a week just to work. I used to get around 570 - 590 a tank using cheap super market diesel.
After mapping and using Shell ( more expensive ) and driving the same It looks like I'm loosing about 35 - 50 miles per tank.
The mapping has totally change the car on throttle response smoothness and get up and go, but most of all you can drive it like it was petrol car ! gear changing, excellaration etc
Hope this helps.