Welcome to ecomodder.
The higher your MPG get, or want to get, the worse some spirited driving will affect a tank.
While the MPG scale may seem linear, it's not.
Sure, 25mpg burns 2 times the fuel than 50mpg,
But every extra mile you squeeze out of a gallon is harder to achieve than the previous one.
With a 63mph average, your speed is too high to get the high MPG numbers.
Even lean burn will burn less at lower speed.
You'll likely be able to get 50mpg.
Maximise the time you're in lean burn.
Try to keep WOT @ 75-80% of throttle
Go slower - except over long distance drives it won't take much longer, if at all.
Extra weight mostly affects city driving.
Once on the highway and up to speed, it doesn't have that much effect.
Try coasting to slow down, even with the engine on, if you aren't doing that already.
Go over the
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com and see what else you can do to help your mpg up.