RE: Windshield wipers
You could fill a 5-gallon gas can, then run the Jeep till it's empty (stay close to a gas station, though), pour in the 5-gallons to get you to the pumps, then see how much more it takes to fill it. However, if you run the tank dry, be prepared to replace the fuel filter, as it may suck a lot of debris out of the tank into the filter.