2000 magnum

As long as you have a heavy 4x4 truck with 35” tires and are hauling a load, you’re not going anywhere quickly or with good fuel economy. You could put a 400 hp crate engine in it and it will be a dog without a torque converter and new gearing.

How are you calculating your fuel mileage? The only way is by getting mileage by gps and measuring the fuel that goes in. Using the odometer and topping off the fuel tank is a wild guess with bigger tires.