I once got 160 miles out of a full tank in my old 1.6 Focus.
Was doing stop/start driving and ragging the shit out of it

First of all I would get it plugged in and see if theres any codes.
If not then replace the lambda, if that don't fix it try driving it without the MAF or MAP sensor plugged in (Focuses had one or the other) and see if it improves.

Probably worth giving it a major service and running it on some decent fuel too!

Once I got my Focus running right I was getting 40mpg+ out of it without even trying.