431 miles, 9.533 gallons, 45.211 MPG
My nine and a half gallon tank fillup cost me $25.16 at CostCo.
This tank is a couple MPG higher than average, because I've been driving with the windows down and consequently can't leadfoot it at 70-75 MPH all the time. When my Civic was built, the speed limit was 55, and I'm betting the engineers who built it made the engine and transmission optimially efficient right around that speed. Of course, air resistance goes up with the square of your velocity too, so that's likely a factor.
It's going to be a long, hot summer. And I'm going to be hating life, commuting an hour each way with no air conditioning...