What is the best octane of unleaded to use?

Regular unleaded, that's what the car was designed to run according to the manual and I've never had a problem with it. No knocking at all. It's an Atkinson cycle engine and I think that changes things a lot.

 

Regular Unleaded as indicated in the manual, that means around here 87 with 10% ethanol.

 

It's been a while since I've seen the Prius manual.
But I ran mine on regular octane...most of the time.

Never had a problem, never felt performance was hindered in any way.
Superstition and ritual, would have me very rarely run a "premium" tank of gas, which was probably just a waste. I can't say I ever really realized a huge difference.

With my Honda Fit, I still follow the same ritual. Which is operating on regular 99% of the time, with only very rare premium tank fill ups, and occasional Techron Fuel additive treatements.
Both actions which I admit upfront may have more placebo or superstitious value than tangible value.

But that's how I "fuel" my vehicles.

 

Indeed. That 13.1 "compression ratio" doesn't work like it does with a conventional gas engine.

Absolutely, positively NOTHING to be gained from a higher octane.

 

13:1 would be high in an Otto cycle engine. As already mentioned, the Prius engine is an Atkinson cycle, which changes the equation. Use what's called for in the manual.

 

I put premium in mine. I makes me feel good. I can afford $2 extra.
I get a Smiley on the MID.

 

You would do a lot more good if you put that $2 each time you fill up into a can and gave it to a worthwhile charity once a year.

Or you could send it to me.

Either one of those should make you feel even better.

 

I do not have a pet hate for oil companies, but I'm not going to give them extra money for a product that's labeled "premium" just to amp up their profit margins.
+1 on finding better things to do with the $2.
Even lottery tickets would be less wasteful because there is a MUCH better chance that we'll see you on TV holding a poster-sized check than there is that putting "premium" gas into a Prime will benefit it in any way - and dot.gov might 'statistically'....or even ACTUALLY spend some of the lottery money on.....education and junk.

This is late 2018.
Car makers can get by with compression ratios of 13:1 (in both Otto and Atkinson cycle motors) without their buyers having to drive to the drag strip to get gas because engines are fuel injected these days and air-fuel ratios are monitored and controlled.
What Is the Atkinson Combustion Cycle, and What Are Its Benefits? – Feature – Car and Driver

 

Octane not important but use of TopTierGas.com can ensure you getting adequate detergent additives. Energy content relates to MPG and that could vary slightly in your area.

Consider what gasoline region you are in, per below map:
https://www.api.org/~/media/Files/Policy/Fuels-and-Renewables/2016-Oct-RFS/US-Fuel-Requirements/US-Gasoline-Requirements-Map.pdf

I believe you are in conventional gasoline area, which means you could see slightly more energy-content variations between stations and between grades, with no particular trends, trial and error testing. Those of us in Reformulated Gasoline areas have more steady diet between stations and grades.

 

I agree, use top tier gas. The stations that sell top tier around me are Shell, Mobil, and Exxon and they are only about 5 cents per gallon more expensive than non top tier. Plus many of you are only getting gas every one or 2 months so it's not that much more for top tier. And don't waste your money on Premium, it's not required.