Keep in mind that bone dry will suck more debris (dirt, rust, water) into the filter than if you gas it up a little sooner. In hot weather the pump motor runs hot when the tank is empty, shortening its life (or so I've heard).
I get 290-310 miles on 13.5 gallons. I can't keep my foot out of its throat though.
I've seen it written that we get more smiles per gallon than miles per gallon.
Honest, I didn't think that one up.