A gear swap should be planned based on what the most common driving conditions are if fuel economy is the main concern. Most of my driving is at 70 MPH and up, so thats why I went with the gearing I have. Others might spend more time on slower highways (50-60 MPH instead of 70+) and in that case, 3.08s are not the bost choice unless you have an older transmission without overdrive.
I know guys that can get 20+ MPG with 4.10 gears (overdrive tranny) but thats keeping the RPM in the sweet spot and driving very gently (cruising no higher than 55MPH).
After all the modding with my truck, this is what I think is a good guideline for economy running empty:
50 MPH : 1500 RPM
60 MPH : 1600 RPM
70 MPH : 1800-2000 RPM
For towing, add up to 500 RPM to those depending on the load. With a turbocharger, 300 extra RPM should be enough for towing for the same effect.
Even 2000 RPM can still be fairly economical when running empty and it seems to be a good compromise overall for MPGs at highway speeds.
If you spend a lot of time in the city, than thats where 4.10s can be good for economy.
As far as gearing meaning more or less power....
As I said earlier, gearing has no effect on power delivered to the road and once you are in the powerband, accelleration is the same after you are half way through 1st gear. However, cruise RPM will be different, and thats where some careful planning is in order to get the most out of any potential gear swap in terms of MPGs.