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If the batteries are just standard starting batteries, they will not serve this purpose well because deep discharge will quickly damage them. You need deep cycle batteries. I pay 14 cents per kwh and have calculated that it is worth it, though the initial payout for the deep cycle batteries and charger was kinda steep and I don't expect to get it all back from the fuel savings. But your 1991 Festiva will probably have an alternator that is constantly charging, unlike my civic with its electric load detector. That means your alt likely uses more fuel than mine. MetroMPG did a test using his Metro, and found a 10% increase, IIRC. Your car is possibly similar to his.
As to range, I don't think you'll get 120 miles out of three starter batteries more than a few times. But you could do what I and a few others have done: install an alternator kill switch. You can flip the alt on whenever you have to go further than the easily available range. When the field windings in the alt are disabled by the switch being in the "off" position, the pulley will free wheel and that represents almost no load on the engine.
Most importantly, research how to disable your alt. I saw a guy wreck his alt be disconnecting the wrong wires and then trying to run the car normally. Fried it, apparently.
Good luck!
james
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.
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