Here's some numbers. I'm making wild guesses as to weights and drag because I'm too lazy to look up more accurate numbers, but it will give you an idea. Remember that the power available at that speed depends on the transmission gear ratios, so rated engine power will need to be higher.
3500 lbs with driver, fuel, dirt, and stuff
15 hp to hold 55 MPH on a level road
85% driveline efficiency
Then the power needed to climb the hill is:
3500 lbs X 0.06 grade X 55 MPH X 1.467 (convert to ft/sec) / 550 (ft-lbs/sec/hp) = 31 hp at the rear wheels.
Engine power needed is then (31 + 15) / 0.85 = 54 hp
Add 3 hp to run alternator, water pump, power steering pump = 57 hp.
Winter 2011-2012 was warm, so average mileage only dropped to 30.1 MPG. Summer 2012 was also warm, so average mileage increased to 37.1 MPG.