What Grade Fuel do You Use?

I do recommend reading:
The Low-Down on High Octane Gasoline
US Federal Trade Commission's Facts for Consumers - "The Low-Down on High Octane Gasoline"

You should use whatever the owner's manual recommends. (For a US Prius, that's 87 octane (regular), under the (R+M)/2 method (as posted at the gas pumps), which is about 91 octane under the RON method.) If your car is experiencing engine knock, then you should go up a grade in octane. (But if your Prius IS knocking, something is wrong with it!)

(To note, Prius in the UK use 95 octane (RON method, which equates to about 91 octane ((R+M)/2 method) in the US).)

Octane is rated differently in different countries... The US uses the (R+M)/2 method, where "regular" is usually 87 octane, and all gasoline is unleaded for air/water quality reasons. Other countries may use just RON (higher than the US method), while some use MON. Few modern countries still allow lead in their gasoline.

Octane is a measure of the fuel's ability to resist pre-detonation (or engine knock).

Higher octane gasoline doesn't mean that it is necessarily cleaner or better. "Premium" or "super" is a mis-nomer. All gasoline sold in the US must meet certain federal EPA clean-burning guidelines. However, some individual brands _may_ decide to put some extra cleaners or do extra refining in their higher-octane fuel (as I've heard of some brands advertising low-sulfur gasoline as only in their Premium line).

A higher octane gasoline actually has lower BTUs (energy content) than a lower octane gasoline, so the only way that you'd see better fuel economy by going up in octane is if you were previously using lower than recommended octane in your car...

In the Toyota Prius, using higher than recommended octane fuel is known to cause check engine lights with engine misfire codes, besides the expected lower fuel economy.

(Ethanol also has a lower BTU content than does gasoline, so you can expect to see slightly lower fuel economy using E10 fuel. The Prius is not a flex-fuel vehicle, so do not use E85 in it (unless you want check-engine lights and the potential for corroding out your fuel system!)

If you live in a high altitude area (like the Rocky Mountains) usually you can use the next octane level down from what is listed in the owner's manual because of the altitude/thin air with no ill effects (except if you come down from the altitude with a full tank!).

Reasons to use higher octane fuel:
1. Your owners manual calls for it
2. Your engine is knocking
3. It's the only way to get low-sulfur fuel in your area (sulfur will slowly poison your catalyst, making emissions worse over time)
4. You like spending extra money on gasoline

See also:
Premium vs. Regular : Car Talk | Premium versus Regular

If you don't have an aversion to using either Mobil or Walmart or any other gasoline distributor for their business practices, then go to whichever you like for price/service/convienence... It is possible that they could get their gasoline from the same place, or that one place has better cleaning agents added to their fuel as opposed to another. Some people prefer Top Tier gasoline retailers. http://www.toptiergas.com/index.html But the main thing is to use the recommended octane of gasoline in your owner's manual...

I also like the History Channel's show "Modern Marvels," the episode titled "Gasoline" for more information...

 

Thanks, but can I ask some people's personal experience such as better MPG with higher octanes? Here in Seattle, we have 87, 89 or 92.

 

Only anectodal. On one long trip, where I burned through three tanks in four days, I had one fill-up on a Shell high-grade gasoline. The only thing I noticed was that my MPGs slipped for the 500-some miles on that tank. The earlier and later tanks showed higher mileage for driving in the same conditions.

I don't have the specifics to give you, as my records are not here with me, but I recall a drop of around 2-3 MPG. Certainly not huge, but also hardly worth the extra coin.

 

Use 87 ONLY!!! Its in the manual!

Higher octanes are bad Egon!!!

 

You get worse mileage with higher octane. Higher octane gas has lower energy density. To get better performance with higher octane, you need an engine designed for higher octane, in which case it will run with higher compression and/or advanced spark. The Prius engine is not designed to do this.

Tom

 

Tom has it dialed! You are only throwing your money away to by higher octane than recommended. We drive across Montana several times a year and the octane there is very low,,, ~85. The car runs fine,,, and the fuel mileage is as better than normal by 1-2 mpg.

Icarus

 

I 'm not trying to pick a fight, just adding anecdotal statement. I try not to use E10 (or E-anything). But I have had 3-4 tanks of E10 because of situations where that's what was available. I don't have my mileage book at hand, but I haven't noticed any significant change in MPG, E10 vs. real gas. I would like to say that E10 gives lower MPG, but that has not been my experience. Neither has it given significantly higher MPG, as far as I recall. You may be right as to the average outcomes, but my experience is different. Feel free to add this to your data, if you want -- or don't; just my experience.

 

My experience is quite to the contrary. I have had only a few tanks where the gasoline was not E-10. Each of those tanks produced higher MPGs than the "normal" up to 10% ethanol gasoline.

 

Thats the fallacy of ethanol. The "gas savings" of ethanol are dubious at best. The real reduction in mpg alone begins to lose the advantage. by the time you take into account the amount of energy used to make ethanol and it becomes a net loser!

Add to that, the cost to food production and IMHO ethanol should not be promoted as green energy!

Icarus

 

Ethanol should produce lower mileage. It's pretty simple chemistry. The amount of chemical energy in ethanol is lower than the amount in gasoline.

Tom

 

Thats 100% correct.:rockon:

 

And 85 octane in the mountains, per the manual.

Tom

 

You will get better fuel mileage in Montana due to the higher altitude, regardless of octane. The lower air density causes less drag, and lower pumping loss in Otto-cycle engines. Not having an Atkinson engine yet, I don't know how they respond.

My '86 Accord, when it was the travel car, normally got 5-10% better FE there than at sea level.

 

For the geographically challenged: Most of Montana is not very high elevation. Compared to Colorado, or Utah or parts of Wyoming it is very low. Missoula is ~ 3900', Bozeman ~4900', compared to Denver's 5200, or 7000 for Park City.

Mountain passes in Montana range from ~3500-6000' a perhaps a bit higher near Yellowstone. Colorado passes run all the way up to ~11,000+' and more.

So,,, I don't think that there is much "high elevation" less dense air advantage to Montana.

Icarus

 

I got better mileage with E10 in my Camry because I could advance the timing with no detonation. I was also able to use 5th gear at 60km/h but I would have caused harm to the engine on the lower octane fuel without ethanol. E10 is 4 cents per litre cheaper. I use E10 sometimes now, I have to go out of my way to get it. It cleans the fuel system a little.

 

For the air pressure challenged: Normal air pressure at 3900' is 13% less than at sea level. When about half your vehicle's friction loss is air drag ... well, you do the math.

My fuel log can see a slight difference in Eastern WA and ID, where gasoline is still 87 octane. MT is better. In WY and CO, wow!

I still drive Otto-cycle engines, which also suffer serious part-throttle pumping losses. Your newer Atkinson-cycle Prius ought to be less affected by this factor.

 

You still have to push the stuff out of the way. The thinner the better.

 

We are at 800 feet. I can hardly walk in such dense air.

Tom