Older aircraft going faster and wasting less fuel?

[Pomo1]

I wonder if anyone can give me some feedback regarding B737- 300.

I fly 737-300 that are now nearly 15 yrs old. They r the most used boeings on Europe. Last week I tried increasing the cruising speed to Mach.767 instead of .75. obvously the optimum altitude for cruise will lower at higher speeds. I tried it on a 4 hr flight and found that i burnt 400kg less fuel than was planned. Can anybody help me come up with some solution? could it be that older aircraft have more drag and need to fly faster?

thanks