I gained about 3 mpg when I installed the K&N CAI (cold air intake), then lost about 2 mpg when I went to 3.73's. So if I can keep my foot off the floorboard it comes out about even. If you don't have a CAI yet, I'd recommend one. You can get the K&N for around $250 and there are others also, but some give so much more air to the engine that you need to reflash the computer to get the full benefit. You don't have to retune with the K&N, although it will help make a little more power. It depends on your budget, a CAI + tuner can run $600. The engine will breath better and produce more power, and it'll compensate your gas mileage, at least partially, for the step up in gears. All this is from my experience with the 3v 4.6L, I think you've got either a 2v or 4v, I'm not sure which, so someone with the same engine may give ou a little better info on the CAI.
Going from 3.27 to 3.73, you'll see about a 12.5% increase in RPM's at the same speed or a 12.5% reduction in speed at the same RPM's. So, if you were running 75 mph at 2000 rpm before, you'd be running about 65 mph at 2000 rpm after going to 3.73. So you'll need to adjust your speedo for the new gears also.
Also, check around your area for speed shops that can install the gears for you. Chances are they'll charge about 1/2 what Ford will want to do it.
