F1vel66a, when it comes to exhaust, you are having to play a balancing game here. Yes, lower back pressure allows the motor to not have to work as hard to push the exhaust gases out, but at the same time, the back pressure is what creates your low RPM torque. So, what can happen is out on the highway, you may get slightly better mileage with say a true dual 3" pipe setup, using an H-pipe, and a 40 series muffler. But, in town, you will have lost most of your back pressure and you are going to have to really get on the gas to make it take off unless you are doing 4,000 RPM tranny drops to get the truck going. Where if you take a step back to say dual 2.5" pipes, an X-pipe, and a 70 (or even possibly a 50) series muffler, you will keep more back pressure which will make the truck more driveable in town and still drop the back pressure at highway speeds to give you better mileage.
I would recommend 2 other modifications if you are looking to maximize mileage. The first is doing the "Gotts" mod or going with an aftermarket intake. This will allow the engine to breathe easier, allowing more air to reach the cylinders and just make things work easier. The other modification is converting over to electric fans. Doing this modification frees up about 6 hp and 8 ft-lbs of torque at all RPMs. So, even though you are not messing with the motor itself (internally), the motor is not having to power other devices, so, that leaves more power to go to the rear wheels. That allows you to accelerate at the same rate and not have to put your foot as much into the gas (therefore better mileage).
Please keep in mind that 2200 RPM is a magical number. Driving with the RPMs above this results in decreasing mileage the further above it you go. I have done a lot of playing with my truck and doing detailed mileage analysis. When on the highway, you want to keep your cruising speed between 1800 and 2200 RPM (closer to 2100 RPM is the best). Also, when accelerating, depressing the gas pedal so the truck naturally shifts at about 2500 RPM results in the best intown mileage. Are you going to accelerate as fast as the car next to you. By no means. But, are you more interested in performance or mileage? A corvette may be able to pull 25 mpg on the highway, but if you goose the car off the line from a red light, the mileage will dip below 5 mpg easily (yes, 5). That is where people make big bucks in how they program the computers to maximize mileage. You can prove this to yourself using 2 tanks of gas. Drive one where you are gentle on the gas and do as I say and allow the truck to shift normally at 2500 RPM. The other tank, get on the gas and let it shift at above 4,000 RPM routinely. See what the mileage difference is. I think you will be surprised.
