car mileage

Hi all,
went for trip down to Nyamup from perth for the school holidays in my Korean SPORts car... normally fill in regular unleaded fuel but decided to fill it with super 98(?!) premium unleaded from Caltex. Normally a fuel tank gives me 510-520 kms but this trip it lasted for 650kms a whopping 25% more...the cost of the reg ., fuel $1.42 a litre and this one costed $1.59 a litre only a 12% higher cost....so what say you ...should I continue with premium fuel all the time?

disclosure

1) no shares in caltex/woolies
2)me not related to the cheneys
3)Korean sports car = Hyundai Elantra manual.