Going up hills costs a lot more fuel than you may think. After all, your car is doing all the work so the driver doesn't notice. The chart shows the power used by a 1995 Ford Taurus engine as it moves the vehicle along at a steady cruise speed on the level, as it climbs a 6% grade at constant speed, and as it accelerates on the level. You can see that depending on speed, going up the grade costs from triple to double the power. Power translates to fuel. To give an idea, a power output of 20 kW (the units shown on the chart) costs you about 2.1 gallons per hundred miles (GPHM). Remember that the chart shows the power output of the engine. After taking into account powertrain losses (in the transmission, differential, etc) it will cost you more than 2.1 GPHM.
The lesson is to avoid hills like the plague. If you can take a slightly longer route that detours around a hill instead of climbing over it and coming down the far side, do it. Figuring out these route adjustments is easy with an instrument like the scangauge. If you have to climb a hill, see if you can rearrange your trip so you do it after the engine is warm. Cold engines are less efficient. The last thing you want is to take a hill with an efficiency penalty. If you have to carry cargo during a series of trips, think about if it is possible to drop it off before taking a hill. If you are planning to move to a new house, think about picking a location where you can commute to work on the level. If you work in a valley, don't live up on the ridge. Climbing it every day could inflate your gas bill!