Earl if you change trannies and such things can be different. I was just pointing out that there is no free lunch so to speak. Not trying to be a naysayer. Gas mileage on the freeway is mostly a funtion of drag and driveline friction. In town is more to do with weight/inertia/gearing/lead foot. If going 60mph takes 50Hp with 3.73's it will take 50Hp with 3.07's. There may be very marginal gains because of the friction of the gears themselves but ignore that for a second. So if you change gears your engine slows down say 18%. Most likely your motor doesn't make as much power there so you have to push down on the gas harder. So you end up using the same gas. Now maybe you have a big block that makes a gob of torque everywhere, then maybe you don't have that problem, you lower rpm and get a gain in less loss from the part aoround the rings and piston that doesn't burn. Maybe you have a small block and you end up pushing the rpm down far enough you are off the torque peak completely and the engine is less efficient. Engines are most efficient at peak torque. As in the amount of gas used per Hp made goes down slightly so you don't want to get too far away from that. If it were jsut as ez as puttin higher gears I would get some 1.0:1's made and quadruple my gas mileage. We all know that won't work well.
