I had 3.55s before the 3.73s that I have now. Go ahead and get the 3.73s. There's not that much difference, rpm wise, on the highway, and I still get in the 20s mpg. I do alot of interstate/highway driving as well. Most of it at about 75 mph. The rpms are just about right. It's nice to not have to shift to pass someone, even in a hurry. With those gears with just a little bit of gas you're around them quite quickly. No downshifting to 4th just to pass someone unless you really have to haul to get around them.
I think you'd like the 3.73s better than the 3.55s in the vert.
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1996 Mustang GTS