"Fuel consumption of any version of these engines was exceptionally heavy. A PT boat carried 3,000 gallons (11,360 liters) of 100
octane aviation fuel, enough for an 4M-2500 equipped boat to conduct a maximum 12 hour patrol. Some 200 gallons [760 l] an hour were consumed at a cruising speed of 23 knots, increasing to 500 gallons [1,890 l] per hour at top speed. Hull fouling and engine wear could both decrease top speed and increase fuel consumption materially."
From wiki article on PT boats and Packard V-12's . WOW