I'm in the US so i don't know the metric to english conversion, but i got poor mileage when i had a 5 speed 2.3L mustang ('91). I actually get close to similar mileage with my '84 5.0L carbureted mustang (difference of about 3 miles per gallon less and 100 horsepower and 140 lb/ft of torque more with the 5.0L). I tried the same thing and was dissapointed with the gas mileage of my mustang when i had it because it was gutless and got poor mileage. Getting a new oxygen sensor may help but don't expect any miracles, it sounds like you're getting typical mileage with yours.
------------------
'84 Mustang 5.0 LX
My car