RPMs will go up obviously.
These cars have a sweet spot for hwy MPG around 1800-2000 RPM.
For example:
285/35/19 rear tire on your car currently turns 1904 RPM @ 70 MPH in 6th gear on the 6R80.
If you upped your rear gears to 3.55, you would turn 2146 RPM, theorectically, at the same MPH.
3.73s would be a big jump and your hwy MPG would suffer, as you are now over 2255 RPM. You would probably lose 1-1.5 MPG on the highway, but city may go up a little bit.
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