1997 GTP lost gas mileage
Before gasohol the car got 27 MPG. When gasohol first came out I used it for a month without realizing it. It was at the end of that month that I noticed my MPG had dropped about 10% to 24 MPG and I began searching out filling stations that sold gas without methanol. I can't be sure that the drop in mileage isn't a coincidence, though.
Also, I now seem to notice a little bit of a flat spot in performance that occurs just before the boost kicks-in. It'* like the boost is coming in late.
I bought the car new and it now has 185,000 miles on it but it still generally runs great and the tail pipes are white.