I don't think you will save enough fuel to notice it. In other words, maybe 2-3 mpg highway. Definitely not more than 5 mpg, and you won't lose mileage ,everything else being equal. Load increases but rpm goes down. One increases fuel consumption and the other decreases fuel consumption. I think that you will save fuel, but it's well within tank-tank and seasonal variance in mpg so it's not very noticable. I don't drive any faster but the car is at 2000 or 2100 rpm instead of 2500 rpm and it's a lot quieter.
The mileage difference is so small that I doubt you will save more fuel than it's worth. I don't have it but from what I hear it lowers the rpm. The car is quieter and since most people drive on the highway at 65 mph and faster, the swap might be for them.
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