John,
I HIGHLY doubt that he'd ever see more than 18 MPG, realistically more like 16.
However, your right about the gas mileage being an indication (along with the failed smog) that the motor fool infection is not working correctly.
It's probably stuck in it's limp home mode. It's probably a bad legend sensor, incorrect wiring during the install or perhaps a bad computer.
I also found this:
Technical Notes:
The installation instructions is missing the information on how to connect the o2 sensor and IAT. The wires are not labeled and described in the instructions either. (Red Fused/ Battery, Purple/Ignition, Gray/Tach, White/Digital Tach)
Problem - Ram Jets going into “limp home” or shutting down completely, but turning the ignition off and then restarting it solves the problem temporarily. This problem occurs when the temp sender is located in the upper radiator tank and when coolant level drops, the sender is not in the coolant. This causes a false “HOT” signal and shuts the ECM down. Do not use this arrangement and place your sensor some place other than the radiator for best results.
Fuel injected motors rely on the temperature sender(s) to decide on how much fuel to mix with the air flow. If the sensor give the computer the wrong value, the motor gets too much gas and runs too rich. Clearly what is happening with your motor, but perhaps for another reason.
Anybody at High Tech can help you, they are good guys. Any competent garage with real SAE mechanics can address a problem like this. You could probably find someone closer to DC. However, a lot of mechanics prefer not to mess with stuff like this because they can make more money throwing mufflers, brake pads and timing belts onto cars.