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I have a Jap import 2005 Ford Fiesta Ghia 1.6L Auto which says premium fuel. I currently use BP Ultimate 98 or is 95 better (I refuse to use ethanol fuel)? It is preforming great on 98 but when I was forced to put half a tank of 95 in it was a lot weaker, especially on the Rimutaka hill road.

I'm surprised you noticed any great difference between 95 and 98. If the manufacturer recommends a minimum of 95, the car will usually run fine on 95. However, if you found it ran a great deal better on 98, I guess the proof of the pudding is in the eating, but there should be no detrimental effect running it on 95.