Best fuel for calais

HOLDEN couldn't tell me how much improvement you get going from unleaded to premium, but they did tell me the fuel economy figure they quote is achieved on premium and is the best they could achieve from their testing. They didn't have the actual performance figures either, but their engineers say the improvement is in the order of a few per cent. From that you'd think it isn't a lot and would lead me to think it isn't worth the extra few cents premium costs.