I just double checked my fuel logs, and adding 1,000 pounds to my nearly 3 ton pickup truck, on the same route amounts to less than 0.5 MPG difference. Personally I'd take the added security of a larger battery. There's nothing I hate more than being stranded in the middle of nowhere with a vehicle that won't start. As a matter of fact, when replacing batteries, I always buy the biggest one I can fit in there.
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