[navylife59]

Premium used to be a higher octane gasoline with detergent additives. Nowadays all the gas products at your higher end stations (Exxon, Chevron, Texaco, BP, Mobil, Shell, etc) come with the same additive blends. El Cheapo stations are usually running straight gasoline with only the Federally required additives for emission control. Most gasoline companies get their fuel from the same pipeline. Chevron, however, is one of the few that has their own private underground fuel lines. The History Channel's show 'Modern Marvels' had an interesting segment on this.

87 octane (non-El Cheapo) is just fine for your tC; auto or otherwise. Toyota has your ECM tuned to operate safely and efficiently. I bet you could run 83 octane no problem. What matters is detonation, aka "Knock". In a hot climate with 100+ days, you wouldn't be able to run 83 octane in your tC. The hotter your Air Intake Charge is, the morre likely it will precombust and cause knock. Also, the hotter fuel will evaporate more readily and cause a "Lean" condition. To combat this, the ECM would have to keep retarding the timing so much that you will loose appreciable amount of power. The may end up with an overly lean condition (long hot idle with a quick transition to WOT) that the ECM cannot adjust for and end up with severe detonation. Higher octane gas would bring you back into a safer range of timing.

As stated earlier by Alex8181, compression has alot to do with what octane rating that you need. 11:1 comp ratio would need 93+ in a mild climate and more in a hotter climate. 8.3:1 comp ratio would easily run on 87 octane even in 100+F conditions. Of course alot of this capability is governed by the ECM's abilities and the state of tune that your vehicle is in.

I have an '05 tC with 9PSI TRD S/C (no intercooling or H2O injection) and she runs on the street just fine with 89 octane. She seems to run better on premium blend gasolines of 89 octanes rather than 93. I have ven run a mix of 70% 87 octane and 30% 89 octane just cruzing on th highway on a 9 hour trip with no problems. The TRD tune, even at the overboosted level of 9PSI, is still very conservative. I will need, however, water injection/intercooling, to boost up to 14.5PSI.