Just wonder if a plugged or pinched off return line would have any effect. Probably not but a brake or two dragging quite a bit would do it as suggested. One good test would be to check each wheel to see if it is warm or hotter than the others after a jaunt. If all are equal temperature they are probably ok. The other thing is you mention car is really fast. That in itself lessons the likelyhood of a brake problem. The simplest thing that can and does radicallly affect milage is a really dirty air cleaner filter. easy to check out as well.