What gas do you use?

I suggest that you use 87 octane, because higher octane is a waste of money. Instead, put your money into a Top Tier gas, so your engine will stay clean.

 

Last year, I used my Graham scanner to monitor the engine torque, rpm and fuel consumption. Using the same hill at a cruise control maintained 55 mph, I measured the engine output as a function of fuel consumption for different brands and octanes:

All gas was bought in Huntsville AL and other areas may have different sources. However, I would expect gasoline sold in the southeast to have similar characteristics. East coast, west coast and even upper plains gasoline are likely to have different characteristics.

In the late summer, I repeated the test to see what the summer blend looked like:


I don't believe there is anything at the service stations or ads that tells us the relative energy content of gasoline. The only way consumers have of telling is to perform our own measurements and share the results.

Bob Wilson

 

Agreed, and that's not what I wrote. My point was that the higher octane fuels have higher energy because of their formulation, not that the word octane itself relates to energy content per volume. Energy content per volume is measured by the "specific energy" of the fuel.

Which is essentially what I wrote in the second sentence of my opening paragraph...

The real issue here, and pertinent to the OP's question, is not what energy is IN the fuel but what energy your engine can get OUT of it. This is the apparent contradiction of high octane fuel; although its formulated to be less volatile and therefore requires a higher activation energy it can be burned more efficiently (assuming of course that your engine/ECU can make use of it - big assumption I know).

The only way to tell is to try it for yourself. If you notice a difference and are happy, fine. If not, don't bother with the extra expense. I agree with Bob about performing measurements and sharing results.

Bob - Those graphs are fascinating, but as an Aussie down under I have no idea what the relative ratings of each of those fuels are. There would appear to be a significant difference in the top 3 plots of the first graph.

The oil companies are a bit coy about the energy content of their retail fuels, only publishing data where they have to. Shell Australia publishes the specific energy of Avgas 100 for example (44MJ/kg) but none of their automotive fuels. Which is great if you happen to own an aircraft but not very useful otherwise

 

A guy in another thread here claimed that he was on a long trip and burned some 83-85 octane stuff and that he got better mileage with the lower octane than with 87. I read somewhere that lower octane fuel actually contains more energy and that as long as it doesn't cause the engine to knock is going to give better fuel-efficiency.

 

2008 manual asks for 87 (RON 91). Higher octane ratings are required for engines which have higher compression ratios.
Independent dyno results have shown that some turbocharged cars can extract a marginal torque gain from higher octane ratings than requested by their manufacturer.

These same studies have shown that gas brand is an extremely unreliable indicator for fuel quality since certain brands share refineries and distribution networks to contain costs, especially in remote areas. Refueling from a station that has a lot of turnover, which is the guarantee that gas is fresh, i.e. has not had the time to degrade.

So running 89 or 91 is largely useless, except if you are an Exxon Mobil shareholder.

 

I just had to do my own empirical test. All that theory is interesting but not enough information has been provided about actual tests with higher octane fuel in this thread. I've done this test on other cars and decided 91 was less expensive than 87 octane in some cases.

As the price of fuel goes up the cost difference between 87 and 91 generally has not changed. There seems to be 20 cents difference between the two grades. When 87 is $1.00 and 91 is $1.20 that is a 20% difference. When 87 is $3.00 a gallon and 91 is $3.20 that is only a 6.6% difference. So if you get 10% better mileage with 91 it makes sense to use it when gas prices are high. For a year we've used 91 octane in our Honda Odyssey based on this calculation.

So I tried a tank of gas in the Prius with 87, next tank with 91, and the third fill with 87. I was on a long trip on mostly flat roads and there was no noticeable difference between the two grades. So I'm fixed on buying 87 from now on.

What I did notice from bwilson4web's chart was it seemed Shell gas performed better than other brands. I tried Shell and sure enough had a 4% improvement. I still have to do more testing here but I will keep track of which brand I'm using from now on. I haven't used Shell most of my life (now 61 years old) because my auto mechanic father found more damage from Shell gas when he rebuilt engines.

jd

 

87 octo, usually shell or chev because that's who i have card's with. If i owned a car with a V8, sure 89 or 91 would be the ideal fuel..but i dont, hence eco prius and 87 octo. Once i used 85 oct in NM during winter, never seen that before except when i was up their.

 

Is BP gas good for a Prius. Just got a 2008 prius.

 

I seem to get about 2mpg better mileage with Chevron than Costco. In my area Chevron does not use E10 but Costco does. That's about 4% difference so I consider that in my decision to buy. If the price difference is less than 4% (app. 18 cents today) I consider the Chevron a better deal.

I always use regular--87.

 

I usually get shell gas. My husbands swears the premium is best. I was getting 52 mpg the 1st 3 fillups. Then I got too low so I had to get gas at some random gas station (76 I think )and I still used premium but now I am at 45 mpg and can't do anything to raise it???? I will be back at Shell as soon as it is gone!!