gallons per hour will depend upon nozzle size: they are rated for volumetric flow so the answer is probably closer to you than you think. The nozzle sizes are:
60 ml/min, 100 ml/min, 175 ml/min, 225 ml/min, 375 ml/min, and 625 ml/min. My guess is that you would run dual 225ml nozzles or some mix thereof that will flow near 500ml/min at max flow.
Snow makes a 2.5 gallon tank for the trunk, and that is what I have in my mustang...