Rough idle/bad mpg is probably not the O2 sensors. Look for vacuum leaks like cracked hoses, open vacuum ports and stuff. Main culprit are the vacuum lines for the cruise control. Second is the lines for the charcoal canister. It may be a zero-buck fix.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
All my troubles are Rover