2003 ZTS - horrible gas mileage - 16.3mpg - HELP

[PWGuy]

I know this thread pops up ALOT around here but I am at a complete loss as to why I'm getting such horrific gas mileage (just filled up - 16.3mpg achieved in last tank) over the last couple months.

I got the car in November 2011 from my father-in-law. I think the mpg at that time was about 23-25 highway or so - definitely room for improvement. Oh, the car is running on stock rims with stock tire size but Blizzaks for tires as it is more of a winter car or for bopping around town. Last I checked the tire pressure was up pretty high - can't recall exact figure at the moment.

Since November 2011 I've replaced:

Fuel Filter (Motorcraft)
PCV Valve (Motorcraft)
PCV Hose Elbow (Motorcraft)
PCV Valve Hose - Exhaust (Motorcraft)
PCV Vavle Hose - Intake (Motorcraft)
Thermostat Housing and O-Ring (Motorcraft)
Thermostat (Motorcraft)
Valve Cover Gasket (Motorcraft)
Air box - mine was cracked (Motorcraft)
Air Filter - K&N
Upper/Front O2 Sensor (Motorcraft)
Spark Plugs (Motorcraft SP-478 - platinum)
Spark Plug Wires (Motorcraft WR-5974)

Other:

• Cleaned the MAF a few months after installing the K&N
• Replaced power steering fluid with ATF Mercon V (won't affect mpg)

Considering:

• Cleaning IAC to help with idle issues with A/C
• Tranny fluid flush & filter change (won't affect mpg)
• Seafoam

I'm not sure when the gas mileage began to drop from 23ish to 16 - maybe the last 3-4 months?

I'm not sure what else to do and am really frustrated.

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