This is my daily 25 mile each way commute which almost entirely along country roads (where 50-60 is safe based on limit point etc, the road is not that flat but only really gentle gradients), though there's a few poor visibility bends and a lot of villages with 30mph limits so quite a lot of speeding up and slowing down.
I don't think there's much in the way of things on the car that can be changed to help economy knowing what has already been done on yours. I think that it's a lot to do with driving style to get the extra bit of economy (most of which the 3L does for you)
Mods on the vehicle:
- Aero mods (FSI winglets on front, similar winglets on the rear and underbody spoiler)
- 6 speed PTW gearbox
- Check all the shutlines and seals on the front
- DEFA heater - though this hasn't been used since April
- Lightweight the car - I have the back seats removed as it's more useful to me in van mode - I'm sure better could be achieved in a 6 speed TDI 75 without the sunroof (but damn I like it) don't keep anything not needed in the car
- Tyre pressures - I'm not running the pressures high (tried that but it induced all sorts of awful understeer) but I make sure that they are at the correct inflation regularly per the label behind the filler
- Lightweight pepperpot wheels and low rolling resistance tyres (Currently on Michelin ePrimacy all around)
Driver mods:
- Let the engine warm up, the TDI drinks fuel if it's cold and you make big demands of it, a working genuine thermostat from Audi is essential to get up to temperature
- Once up to temp use the highest gear possible (but without lugging the engine/turbo, change down before this starts to happen) and drive gently - you can still make progress driving like this - I find that on my commute I am still catching up with other people on the roads and am very rarely overtaken or tailgated (except in the 30 mph limits where I don't think anyone else actually sticks to 30!).
- Knowing the road and anticipation - things like not accelerating hard when coming out of a bend knowing that just a bit after there's a lower speed limit, use hills to decelerate if there's a reduced limit at the top.
- Anticipation as above, I don't find that I use the brakes that much at all as I use the engine braking so the fuel cutoff is on (also make sure that they are not binding, my rear drums were binding as the handbrake mechanism was totally seized when I got the car). Note: I only slow down in gear like this and don’t sequentially downshift to use the gearbox to slow down as this to my mind is not sympathetic to an old car. If I need to slow down a lot say from 60 to 30 then I’ll lift off and slow down to around 45 or so in gear then use the brakes for the last bit down to 30 and go straight to the right gear for 30. Usually this is a block change from 6th to 4th.
- I do overtake if appropriate to make progress and on these occasions (probably 3 or 4 in a week) I run the revs out all the way to give the engine and turbo a chance to run at high speeds and hopefully keep everything cleaner.