2002 Gmc 2500hd With 8.1l Getting Only 8.4 On Hwy

[REV353]

I have had this 02' for ten months now, I have changed the motor,tranny and front and rear Diff over to Amsoil, and I just can't seem to get to the 10mpg plus mark, I have read that 11-14 on the hwy, why not me?. Looking for help.

Next, my blower motor or vents inside barely can blow out a match, each number 1-5 does work and changing it from floor to defrost or dash vents works, is this normal for this truck or year, or is there something wrong?.

Last. being it was -28f last week I seen my exhaust has a small leak around the back door area, I will be looking to fix this problem and hopefully installing a better system, I am past the loud dual exhaust bouncing off my trailer, it's kinda nice being able to hear a fly fart now. Not looking to break the bank or gosh knows needing any more power.