i concur with most all of what has already been said. but if you are having a lot of down shifting and the engine is lugging you could possibly see an increase. not much but it sure would reduce transmission heat and friction withing the torque converter (some refer to this friction as fluid shearing). the gear stated is not enough IMHO i would say you need a 3:73 or 4:10 especially if you have 2 OD ranges. it will however decrease you non towing mileage without a doubt. But i would be more concerned with transmission life than fuel mileage either way. I have installed dozens of gears and out side of towing you will not likely ever get better mileage with a higher ratio. YMMV
NOTE: you should see the things that happen when the fluid temp of a transmission goes over 200 deg/ valves stick, fluid temp runs away, fluid is vented at an alarming rate, sometimes onto the catalytic converter and then you have tons o smoke and maybe even a fire.