as engines get older the throttle body gets clogged and dirty.. cleaners can do so much but there will always be build up that will get worse. This will affect idle cold idle, make your car shake a little run rougher..

most mechs over services will adjust the throttle to compensate for this.. this will include adjusting the little screw near the throttle body i think its called the cold idle adjustment or something.. basically lifts the idle slightly. It feels great driving but then affects your consumption. Yours is so old that chances are that it has been adjusted to max. The best way to fix that is to actually open the throttle body up and clean it out completely.

There is a possibility that this is your problem as its common amongst many cars.