Hey Will, if you wouldn't mind doing a couple of runs with your current tires so that you have something to compare them to when you get your new tires.
I would assume that at 50 mph on the flat, tires at sidewall max, and it being around 70 degrees outside you should be getting around 46 - 48 mpg.
Granted I don't know what type of tires you have up front and that can play a rather large role in what mpg your getting so I guessed a little lower then what I got in my
A - B - A tests I did the other week.
That reminds me I need to post those results.
Hopefully break in period on your new tires isn't that long. I've heard a lot of people will get worse gas mileage for the first several hundred or even a couple thousand miles till they are broken in.