Sure letting the car slow down as you climb a hill will get you to the top on less gas.

But your car is going slower at the top, so it has LESS kinetic energy.
Therefore, it will take more gas to go down (even if the thing says 120mpg), or you will be going slower as you get to the bottom of the hill.
So you will need more gas to get back up to speed than you would if you were going faster at the top.
Just because you are going downhill at the moment and that meter says 120 MPG doesn't mean the laws of conservation of energy are on hold.
The key not just how much gas you are using at the moment but also the kinetic energy (speed) your car has at the moment.

Look at it another way:
Ignore the uphill part for the moment.
Imagine two cars side by side at the top of the hill at the same moment.
One is going 10 miles per hour the other 30 MPH.
They both go down the hill the with the goal of both going the same speed when they hit bottom.
Which one will use more gas?
The slower one of course.

Again, I will try to do some tests on that bridge.
Some variables I can't control will be:
1. Any tail wind will help more going uphill than downhill since the hill blocks the tailwind on the way down.
2. Any headwind will hurt me more on the downhill side than on the uphill side since the hill blocks the headwind on the way up.
3. I don't know the exact elevation at the beginning and the end. To be fair they should be the same elevation.
4. The car's MPG indicator refreshes every 10 seconds. A one second or less refresh time would be more useful. A lot depends on exactly when I happen to look at it.

I could be wrong; it's happened before.

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.