Help our hosts settle a marital dispute

If the car does not have an MPG readout, and you really want to settle this, you can buy an OBD computer that will give you a mileage readout.

That said, it depends so much on the car and the size or steepness of the hill. My mid-size car gets about 28mpg on a flat highway at 55 mph, but less than 5 mpg climbing a moderate hill starting from a stop sign. For example, assume a small modern car that gets 30 mpg on the flat. It might get 10 mpg climbing a hill and 50 mpg going down. In which case, 1 mile up and 1 mile down uses 0.12 gallons and 2.5 miles on the flat uses 0.083 gallons, for a savings of 10 cents per trip. If it only got 5 mpg going up hill, you would save 38 cents on each trip if you stay in the flat.