Low gas mileage

I have an 1987 Subaru GL 4WD, 1.8 S#jf2an53b8hf457939. I took it into the garage and they check her out on the diagnostic machine nothing showed up. Mechanic also took her out for a test drive. He checked compression cylinders were between 120psi to 135psi. He check valve timing, checked ignition timing repaired brocken vaccuum lines. Ran on scope. The wagon is still getting about 20 per gallon. Any ideas on where I should be looking to improve the gas millage? Thanks Bob<