I recently took my 2001 Subaru Forester in to a local (trusted) shop for it 90,000 mile service. They tuned it, replaced the front brakes and put in a new wheel bearing. The car drives smoothly, but ever since the service I have experienced about a 50% drop in mileage, from 20-23 to 10-12 mpg. I took it back to the shop and they checked it over (for almost an hour) and found nothing wrong, either with fuel leaks or the tuning specs. What could be responsible for the big loss in mileage?
With that much mpg drop, I would seriously suspect a dragging brake. Drive the car a short distance, and let it coast to a stop without touching the brake pedal. CAREFULLY, check the wheels and brakes. If one is heated, that is the one which is dragging. Inform your repair shop. It might be caused by a stuck brake caliper piston, or an internally blocked brake hose.
How did you measure the mileage? Are you using a on-board computer?
After the rear disc brake pads were changed on my 2000 Blazer (~80), both calipers started dragging. The mpg went down and the truck felt sluggish during acceleration. The calipers had to be replaced.