mitdr774
also a dinner prick
While the turbo could possibly pick up some MPG with a premium gas the 1.8 should see no difference at all. If your MPG went up it is more likely that the premium has a lower percentage of ethanol. Gas with less ethanol is a sure fire way to get better mileage and premium often does have less than the 87.
With the compression ratio the engine uses premium would be much better for it than regular.
I know how ethanol affects economy and the station i use told me that as far as they are aware all grades of fuel they dispense are the same ethanol content. It varies throughout the year, but they said that all grades are the same content at the same time.