Given the fact that you have said you will be on schedule either way...
Whatever speed and altitude gives you the lowest amount of fuel used per nautical mile (or whatever units you like) covered will give you the most amount of fuel at the other end. This is known as specific ground range (SGR). You can work your SGR out by dividing your fuel flow by your groundspeed. At the speed and altitude this figure is the lowest is the one you want to fly at.
Flying at this speed (S.L.O.W) is not always operationally desirable as there are other factors such as time critical costs (crew, maintenance etc) and schedule that may dictate that the actual minimum cost of the operation is improved by flying slightly faster thereby using more fuel, as pointed out by the man most Cathay pilots would not buy a beer for, 411A.