Like you I had a manual TD4 before getting my present auto-box.
I undertake several really long journeys (2000+ miles) several times a year and made the conscious decision that I had to reduce the cost of them. I was using over 250 litres in a week.
I did lots of little things, the EGR by-pass made a big improvement, a little, perhaps psychologically, was gained by changing the pcv, but the single biggest transformation was the Synergy2 box.
I am in no way a salesman for Ron but his box transformed the car.
I would guess that most buyers are looking for improved performance - well they will not be disappointed. However if you then set out to drive the TD4 gently and economically, there can be a huge reduction in fuel costs.
The great temptation is to have fun with, and exploit the new found acceleration. It's possible to overtake in places where before you had to stay behind and wait for a longer gap etc. If you use that new power you will possibly get even less mpg.
Check Ron's website - there are several different boxes depending on what your priorities are. I have a Synergy2a+ with dash switch.
I'm sure you could improve on 24mpg but everything else about your car must be in tip-top shape too. Air-Filter, brakes, tyre pressures, oil etc.
City driving is the thing that ruins my mpg figures. You pay for the convenience of the auto-box in heavy stop-start traffic.
No guarantees, but hope this helps.
