After more thought, a possibility is a bad ESC (knock) sensor or two.
ESC activity over a certain threshold (there's always a little noise from normal combustion events) the engine "pulls out" spark which is to say a retarded timing from MBT is commanded.
You can think of spark advance similar to a pendulum...advance and the heat stays in the combustion chamber, retard, and you throw the heat down the tailpipe, essentially wasting it. The waste isn't necessarily all bad as this is a strategy to recover from ESC activity.
However...it does not help fuel economy...which was the original symptom.
Also I am not aware of any self-diagnosis strategy for the ESC sensor other than perhaps voltage high (short circuit) or voltage low (open circuit).
A quik trip to Autozone would confirm or dispel the notion of the existence of diagnostic codes which are "hidden" by a bad SES bulb.