Based on what you say above, the following should help:
1. SLOW DOWN. You will get NOTICEABLY better mileage at 65MPH rather than 75MPH. (Remember, the power required to push the car thru the air goes up as the SQUARE of the speed, and at 75MPH, most of the power you are using is in pushing the air out of the way.)
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Let me clarify my point a bit about speed versus fuel economy:
If you slow down to 65MPH from 75MPH, you need only 75% of the power to push the car thru the air that you did at the higher speed. YES, you read that right, only 3/4 as much.