Let's get back to the original discussion point. Many people swear that their car runs noticeably better on premium fuel, even though their car manufacturer recommends regular. Hey, they don't call it "premium" or "super" for no good reason, right? Besides, the guys at the racetrack use fuel with a very high octane number, so it must give higher performance. The term "high-octane" is even used in daily conversation, for example to indicate strong coffee.

To sum my conclusion up in one sentence: No, it is highly unlikely that your car will run better on a higher octane fuel than recommended. It is even conceivable that higher octane fuel might decrease performance...

Anyway, let's try to see if there is any conceivable way that premium gas might give better performance after all. Since we know that regular and premium have different chemical compositions, maybe premium has a higher energy content. That could give you more horsepower and fuel efficiency.

Unfortunately, it is nearly impossible to find out what the exact composition of gasoline is (I wonder if the oil companies themselves know). The first problem is that gasoline consists of literally hundreds of different compounds (hydrocarbons).
Then there is the matter of additives added to the fuel. Although many additives are present in minute concentrations, and therefore cannot make a significant difference with regard to energy content, the so-called oxygenates actually are present in appreciable concentrations. Unfortunately, oxygenates (added to reduce harmful emissions) are added to both regular and premium fuel. But probably not in the same concentrations. Oxygenates have the side effect of increasing octane number, so you might speculate that there are more of them in premium than in regular. But that seems to be hard to determine.
Then there is the difference between summer and winter: the composition of gasoline varies with the seasons.
Finally, it is almost certain that different refineries will produce different gasolines.

But, let's see what we can find out anyway.

Gasoline is a mixture of hydrocarbons that have carbon numbers typically varying from 4 to 12. Huh.

The way to get a higher octane number for premium fuel is to change the composition. According to this paper by Chevron, premium gasoline has a higher average number of carbon atoms per molecule than regular gasoline. So, that would suggest that it would have a higher energy content (or heating value), since more carbon atoms typically means more hydrogen atoms as well (the things have to bond with something), so there's more stuff to burn. However, there is still an unknown factor: since gasoline is sold by the liter rather than the kilogram (or by the gallon rather than the pound, for all you metrically challenged folks out there), we need to know the density of premium fuel as opposed to regular fuel as well. You see, if premium would have a lower density than than regular, it would offset the higher carbon content. Admittedly, this is not very likely since hydrocarbons with larger molecules tend to have a higher density. Premium appears to have more aromatics in it, since they have a high resistance to knocking, and if we take benzene for example, we see that it has a higher density than for example hexane (see this table).

Of course, premium could still have a lower heating value than regular (quite a few sites on the Web seem to suggest this). More aromatics could be the cause of this. It seems to be quite difficult to find the heating value of these compounds. No matter.

Then, especially in the USA, fuel is often reformulated. As I understand it, this means that oxygenates such as methanol, ethanol, MTBE (methyl tertiary butyl ether), and ETBE (ethyl tertiary butyl ether) are added to the fuel.

This has the effect of increasing oxygen content, as well as increasing the octane number. It also lowers the heating value of the fuel. So, conceivably, premium could contain more oxygenates than regular (since oxygenates increase octane number), so the heating value of premium could be lower because of this.

Again, it seems impossible to find out exactly what type of oxygenates and how much is present in reformulated gas. So it seems that with all these unknowns, we cannot determine if the heating values of premium and regular fuels are significantly different. So, do we need to throw in the towel?

No! The good folks at Chevron have done the hard work for us. This document states:

"Traditionally, premium has had a slightly higher heating value than regular, and, thus, provided slightly better fuel economy. The difference — less than 1 percent better — is not large enough to offset premium's higher cost. The difference is likely to be less or nonexistent between grades of reformulated gasoline."

So, here we go. Less than 1% difference in fuel economy. Although not mentioned, it seems reasonable that a higher heating value could also imply higher performance. But no more than the aforementioned 1%, obviously. And it seems impossible that anyone could objectively experience this difference just by driving their car.

Note that the price difference between regular and premium is at least 10%.

So, can we come to a final conclusion? Not until we have examined the following argument: "My car is equipped with knock sensors, and my engine management system adjusts the ignition for different octane numbers". That seems to suggest that some cars can make themselves run better on high-octane fuel. So, we'll have to see if that makes any sense.

Next: Octane Number And Ignition Timing

Previous: About Octane Numbers, Engine Performance, And More...

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