I'm pretty sure about the force of gravity being virtually the same as you go uphill downhill or flat.
Draw a force diagram.
If it takes x additional energy to overcome the force of gravity to climb a hill, then gravity will contribute exactly x on the way down.
It has to be a wash.

Anyway, I've driven over this particular huge bridge several times.
Of course my MPG drops by the time I crest it.
But when I get to the bottom I seem to recall that it has returned to what it was before I approached the bridge.

This warrants several delibrate careful tests.

I'll be back in touch when I get data of 10 or 20 passes over the bridge.
I only take that bridge about once or twice a week (it is my alternate route when traffic is bad) so it will be a while.

BTW I have about 4400 miles on my car but the database says 3800, not sure why.

Need fewer troops to support.
Drive a hybrid.
Best tank 71.65 MPG.

Before this car I spent two years learning hypermiling on my 2004 HCH1.