I don't think the moosing fix is causing your mpg to drop.
I've noticed myself that I've "lost" a good 3-5 mpg since November'ish -- which I put down to "them" introducing 'environmentally friendly' stuff into the blend.
I tried switching gas companies (Hess / Citgo / Sunoco) but each tankfull of 87 octane produced the same results.
Am getting approx. 17mpg whereas I used to get a good 24mpg during last summer. (Yes, the car is in good condition - wires, plugs, air filter, etc. etc.)
With the price of gas here in NY - I can't wait until they STOP putting that crap in! -- a $50 fill up only to get a driving range of 200 miles is pretty disheartening!
Gerry
