92 f150 4x4, 302 auto/OD fuel milege problems.

recently purchased 92, f150, 4x4, manual hubs, dual tanks, 302 Auto.
with OD .. getting about 24L/100KM fuel consumpsion.. about 11mpg ..
!! looking for a little more.. wondering what to look for / clean/
replace. ..

already did air breather, air filer, pcv, plugs, cleaned IAC with carb
clean.. and lightly cleaned the throttle body.. (IAC cleaning due to
poor idleing.. its ok now) .. plugs actually looked a bit on the lean
side, but ok..

only thing that needs to be repaired is an exhaust leak.. which is
can't find.. was told it may be the thermator pipe between back of
heads.. ? no smog laws to infere with me removing and blocking it
off.. but have to make sure thats it..

O2 sensors actually look new.. going to remove them and test them..

anyway.. anything else to add.. I'll check it out.. would like to get
15mpg+ .. typical days here are about 3-10 deg celcius right now..

thanks..