I have been trying a tank of mid-grade gas, 89 octane instead of 87. It cost ten cents more per gallon, but my highway mileage w/o AC is 60 or better. Before it was 56 max. But the car is only at 2400 miles.
My drive is perfect- 38 mi. commute on flat coastal roadsm and a few overpasses that HELP my mileage!

Peter