Salute!
For some reason I always thot the Spey would have better gas mileage than the J-79. Amongst the colonists, the F-4 was well known as a gas guzzler, especially from about 1966 to 1973. That was during that miserable fracas where many of us made our bones.
Funniest story we had in the SLUF community was a radio call at Udorn when one of our flights had to land there and refuel. Bear in mind we were flying the Spey motor, minus reheat. About 14,500 lb thrust at mil, and awesome TSFC. So the guys get to the rwy and hold. After 5 or 6 minutes the tower asks about their fuel abort time. Huh? Seems the F-4 J-79's were so bad it could not even get to first refueling point for the mission. So our naive flight leader asks what that was all about. Because we had over 9,000 lb internal and two 300 gal tanks, the calculation was tough, LOL. From Udorn to Korat took about 2,500 pounds, maybe 3,000. Burning about 500 pounds per hour on the ramp, our flight lead told the tower we could hold for maybe another 8 or 9 hours. If I had been him, I woulda ordered pizza!
The only time we had to refuel at the outset of a mission was during the Christmas blitz when they took away our drops and increased the number of MK-82's. So we hit the tank on the way to Hanoi, but came home at 40K, burning about 1,500 pounds per hour. That was in your modified Spey that Allison built for us. We still had two Winders and two ALQ ECM pods, plus the drag from the MER's. Always comfortable in that plane WRT fuel.
Gums sends...