In the mountains you will burn about an additional 1/3 to 1/2 gallon for every 1000 feet you climb. So during the few minutes of the climb your fuel economy is going to drop to half or less. You get much of that back when you coast downhill especially if you can do that without using brakes or engine braking, but that's often not possible. I get killed on fuel economy crossing the Appalachians because I usually take the back roads and there are so many ridges and the downhills are too steep and twisty to avoid braking. I usually get about 10-12 doing that.