What were the "Many other things fixed" recently? Did the car get better gas mileage before "Fixing" it? Could be a stuck EGR or PVC but this would make the motor ping. Have you checked all vacuum hoses and their routing? Is it hard to start or does it lack power? Could be a clogged catalytic converter. Does it show any other unusual symptons, like overheating, stalling, backfiring, or morning sickness? My experience has been that when my car is not getting such good gas mileage, it is also not running too good. Does the check engine light come on? Have you checked to see if any codes are stored in the computer? Is the car an automatic or five speed?
As you can see it would help to have as much information as possible to narrow down the problem or at least help you look in the right direction.
Do you only drive uphill? Pulling a boat? Against a head wind? While dragging your feet?