hmm, you may have similar trouble i've been to with both cars
and no minor tune up was able to get the MPG back in full, so:
check if your brakes (both front and rear) are not dragging.
to check fronts, you need to jack up whole front end up and put it on jack stands (safety precaution). shift tranny into Neutral (either shut off the car in this gear or use the override button by shifter to do it after car is shut off). then spin each of wheels by hand and see if they can do at least one revolution.
to check rears you can have one wheel off ground at a time, make sure to NOT pull the e-brake up. use wheel chocks and Parking gear to secure the car on flat level surface.
spin each of wheels, they should be able to make at least 1-2 revolutions when spun by hand.
if you suspect brakes to be dragging, remove the suspected wheel, unbolt caliper and hang it or rest on dust shield and/or strut tower. also back off the brake pads from rotor so they do not interfere. you caliper slide pins may be getting sticky and fail to back off the pads pressure from rotors.
spin the wheel again, if it spins nicely, then you found the culrpit, if not then maybe your wheel hubs bearings are the problem (bearings slowly getting seized up). bad bearings usually cause more roll resistance at low speeds and still can be good at higehr speeds (car's inertia overcomes the roll resistance after passing some critical mass/speed).
in rears, don't forget that parking brake may be set too tight and cause the drag on drum shoes (applies to both rear drums and rear discs).
more info with pics and videos here (long thread):
12 MPG city in Winter, 1mz-fe
good luck!
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