APV poor fuel consumption

My 1991 APV with 150,000 miles, V6 3.1 litre motor used to get about 400 miles on a tank of gas and suddenly dropped to about 250 miles. I have had a full tune including plugs, ignition wires, air cleaner, timing etc. it passes our government exhaust emissions test. I replaced the O2 sensor. Tyres, alignment , brakes all OK. It starts and runs well.

Has anyone any suggestions?