I guess I should explain in a bit more detail on why ANY electrical load you put on the car will make you burn more gas.

In a vehicle, NONE of the electrical energy comes from the battery. The battery is simply acting as a very large capacitor, and its primary job is to simply start your car. ALL of the electrical energy comes from the alternator. (Even the energy to start your car comes from the alternator, since the alternator charges the battery.) The engine turns the alternator. Thus ALL of the energy comes from the engine. Gas is burned to turn the engine. Thus ALL of the energy comes from the burning of gas.

Here's the progression:

Burn gas → turn engine → turn alternator → electrical power

When you draw current from the alternator, the alternator will become harder to turn, and the engine will have to burn more gas in order to maintain the same RPM. The more current you draw from the alternator, the harder the alternator is to turn.

But again, NONE of these gas-saving apparatuses work. N-O-N-E.