I'm considering to have my car engine chip tuned. I'd like to increase the fuel efficiency of the engine. Increasing the power of the engine would be only a secondary advantage of doing so. Most of the chip tuning companies in my country offer tuning for max power or for max fuel economy.
My question is this: is there anything like economy car engine chip tuning that really works? How much improvement can I expect or what are your real-life results (if any)?
I know there are other ways of increasing fuel economy (mileage), but this is an important one. There is a general consensus that bigger and especially diesel (turbo) engines are better to tune. My car is a 2007 VW Golf 1.9 TDI, but I wouldn't limit answers by that.