Well, an engine management system will definatly help you don't "need" on to get good
fuel
economy. It's a little bit of a misnomer that you can either tune for power or fuel economy. True, things like HUGE injectors and sometimes a High Flow fuel pump can bring down fuel economy, but I still believe you can get good fuel economy on the stock ECU,, and here's how
The stock ECU (well with a daughtercard of some sort for bigger injectors, MAF, ect), has two modes, open and closed. During open loop (I hope I don't mix these up). The computer is checking signals from the MAF and O2 sensor, and probably a host of other sensors. It is trying to achieve the optimal A/F mixture at all times. This is usually as steady state driving conditions such as cruise. The computer doesn't care if you have a
turbo that can put out 600whp (at least not at cruise). It just cares if you are running rich or lean.
On closed loop, (Full throttle, start up, and a few other conditions), the computer goes to the maps and doesn't care about A/F ratio. It does whatever is in the maps. It's at this time when you "tune" for power, and yes, you
mileage will be horrible if you romp on it all the time. The more power you make the worse your mileage.
I have a 98 240sx w/ a 98 Sr20det
engine with for now the stock T28. I have the usual bolt one (550cc injectors, Z32 MAf, chipped ECU, Walbro, FMIC, Full 3" exhaust) and I get 29-30mpg on the highway. In fact, I get over 30mpg if I am doing over 75 mph constantatly. I can't figure that one out except maybe I am driving where the engine works more effeciently. I can run up to 15psi of boost with no probs. So it makes plenty o' HP and still get civic-like mileage..
Hope that helps