unexpected low mileage on 2000 Ford Ranger

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
I haev a manual drive and usually get 19 - 20 miles per gallon... all of a sudden it 13-15 miles per gallon. Air filter is clean, Tire correctly inflated, no change in driving habits. I only use name fuel and change my oil every 3000 miles at the dealer. There is some chirping sound when I start it in cold weather, but stops in less than a 1/4 miles and doesn't happen at all in the summer.
(1) Answer
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if your mileage is up around 100,000 you may want to consider an oxygen sensor. they tend to get lazy over time and that will effect you mileage. also, have an injector cleaning done. injectors not spraying correctly will result in poor mileage.

I've done some research on my own, but thanks to a Ford Parts Dept guy I found out that there are four oxygen filters on a Ford Ranger 2000 XTL (6cyl 3.0) 2 in front and 2 in back of catelytic converter. The cost for Ford Parts $340.00. BUT the pairs guy told me the engine light will go on if there is a problem with the sensors and my truck passed the CA smog test with no problem. So I ran Techron twice with fill-ups with no affect, so I'm down to a getting my fuel filter replaced (it's been 18,000) and maybe the PCV Valve. Then to spark plugs and wires. Any suggestions.
I got new shocks right before I started getting low mileage, would that have any afect on fuel consumption?