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Driving that fast is pretty hard on gas mileage. I understand commute time is a factor as well, but the more you can slow down, the more it will help your mileage. The sweet spot for steady state driving is the minimum speed you can maintain in your top gear. I'm guessing 35 or 40 wouldn't be too safe (or legal) with a 70 mph speed limit.
Along the same lines, aerodynamic improvements may have a bigger impact for you than most others here. Air drag increases significantly at those speeds.
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