Modtech question

Modtech, How are you at figuring Gear ratio's? 89' F-150 4X4 5.8 bored 30 over everything else stock. It does have a towing package from the factory and no overdrive in the auto trans.. I changed the tire size to 31X10.50 - not much from factory. I'm running 3.55 gears and only get about 10 MPG. I do most of my driving on the road. Should I be getting better MPG? I understand that the 65 MPG / 2950 RPM (high RPM)is due to a worn torque converter. any suggestions on this truck?
ervinj