I got better mileage on the street just from adding a turbo on my 2000 4.7 Dakota.

But I will be the first to admit it is because I drove it different. The turbo would spool up and come on like a light switch with any more than 1/4 throttle applied... so I pretty much drove it at 1/4 or less unless I was going WOT. Without the turbo, I would catch myself leaning into the throttle at 1/2 or 3/4 all the time - which is where my mileage went. I don't doubt that the turbo aided efficiency even when it wasn't at WOT - but I could buy a whole bunch of fuel for what the system costs.