ok serious post time , most economical way is to keep the revs as low and the throttle as closed as posibel without labouring the engine.
so for the Z shifting at about 2k and drving real sedatly , although this still means youl go about as fast the rest of the normla cars about.
how ever i give you about 15-20 mins before you just say odd it i paid for a fun car , i pay fun car tax fun car insurance etc etc and just boot it !
i get between 27 and 30 mpg on my comute depending on if im able to open her up in the dual carrige ways or not !
heck was stuck be hind some **** on the way home so just chilled and it now reads 35mpg ... spare gas for tomorrow