I have a 96 ram. I just changed the plugs wires and cap. And I'm still getting 200 miles to a tank. There is no codes.

 

Have heard a lot that those have bad gas mileage. Not heard any numbers so dont know how to compare to yours but maybe you could have access to a wideband and see if it runs rich?

 

Tons and tons of factors at play on an older truck like this one. First you need to make sure everything is working correctly. No vacuum leaks, no dragging brakes, no plugged up cats, no leaky injectors, etc. Couldn't hurt to do a proper seafoam treatment.