O/T: Mileage Calculations

Earlier this month I drove my '64 Galaxie, with my rebuit engine, to Georgia and back for a total of about 1200 miles. Gas mileage averaged 12.3, which I thought was not too bad for this new setup. I now have slightly less than 1500 miles on it.

I think next summer I'm going to finally drive it to California (where it was born) and back (hopefully!), but think perhaps I should consider changing the rear. I'm running a 3.70 posi now. Is there a formula that I can use to determine mileage if using other gears, like 3.50 maybe? Would an open rear make any difference? Don't want to go to high, but better mileage might be nice on a 6000 mile trip. I'm running a toploader.

I could use the savings to buy more beverages on a beautiful California beach while fighting off the beach babes I hear abound there!