I forgot to ask: Are you using a driving style that would net a higher than normal mpg? or are you just driving the car like a regular car and shifting when the light tells you?
Given the fact that the FE of the VX is greatly affected by temps could be a factor at this time of year. But if you are driving the car without much FE technique the car is actually getting the mileage it should be.
The other things to look at now:
De-carbonizing the motor/intake manifold and EGR system.
Checking for proper gap and spark plug type (all listed in the VX lean burn monitor post in General Tech)
VX Lean Burn Monitor
check condition of brake components for dragging
check condition of CV joints
Check air pressure in tires
That's all I can think of at the moment.
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