it seems as if our cars are quite sensitive...
i too have a proECM and in the city i'll get 18mpg too... but that is the dead city... around my parents home (suburbs) i get about 22-25
on the high way (after a bit of HW driving) i've got up to 30mpg
so the proECM did help... but not in the city
you may be ok
rug