The myth about premum is that, because it costs more, it is better. Unless you need the "octane" rating for the particular vehicle to prevent "pinging," it is NOT better. Gasoline is composed of hydrocarbon chains of 6, 7, or 8 carbons linked together. Octane means the mix ratio contains more 8 chain carbion molicules. Higher octain means that it will not "pre-detonate" under higher pressures under higher compression ratios. It does nothing for lower compression engines. Under usual conditions, I do not purchase premium because; It is a slower seller, which means that it is less "fresh" than regular and has been in the underground tank longer to collect more water through condensation. To achieve higher octane, premium has a larger percentage of ethanol. If you look on the pump at the gas station, there is a sticker saying that it can contain up to 10% ethanol. Ethanol provides less mileage and collects condensation water more than gas without or with a lower precentage of ethanol in the mix. Ethanol must be used, since MTBE as an octane booster (methylbutyltertiaryether) was banned in 2005 in many states. So, regular is better functionally and financially in any model Prius. If you look at the gas pump, you may not see an "ethanol" sticker on the regular digital indicator of the pump, but you do for midgrade or premium.