[PaleMelanesian]

Actually, no. Our cars are about as close a match as you can find from different makers. According to Edmunds.com, your Fiat is just barely lighter and about equally aerodynamic. (2363 lb vs 2490, cd 0.33 vs my 0.34). They're both tallish hatchbacks, with the drag that comes with that basic form. The Fit is also slightly bigger, making the cdA worse (66.7 in wide x 60 in high vs your 64.1 wide x 59.8 high).

It could be that your engine management runs rich for a while after a restart or something like that, making the shorter eoc cycle at highway speed less beneficial.

What is your highway gear ratio like? I'm all the way up at 2850 rpm @ 60 mph, so anything I can do to minimize that, like p&g, is a huge benefit. A lower cruising gear would close the gap and reduce the possible gains.