Which Grade?
As gasoline prices climb higher, buoyed by strong demand and high oil prices, many consumers who have habitually bought premium or midgrade gasoline will wonder if they should switch to a cheaper grade to save money. The simple answer is yes, provided your engine doesn’t “knock” or run on after you turn off the ignition. If you are interested in the more involved answer, then read on.

There are many reasons why consumers choose higher octane gasoline, some accurate and others mere myth. Here are the most typical:

• I will get better gas mileage on premium. This is rarely true. Premium gasoline actually contains a bit less energy than regular, so you might even end up getting slightly poorer mileage.
• My car runs better. If that means it knocks on regular but not on midgrade or premium, then you are right. Otherwise, as long as the gasoline is being smoothly combusted, you won’t notice any difference, especially with modern engines that sense how the engine is running and make minor adjustments continuously.
• My car accelerates faster on premium. Not unless it knocks on regular, or the car's owner's manual specifies that it is designed to operate on premium fuel.
• Premium gas has better additives and will keep my engine cleaner. This used to be a bigger concern, before the EPA imposed regulations requiring all gasoline to include detergents and other agents to reduce pollution from dirty engines. Some brands still put a bit more additive in premium. If you think you need it, it’s probably a better deal than buying regular and dosing it with store-bought additive.

Now, in addition to the obvious cost differential, there’s another reason to buy lower octane gasoline, and it’s one you won’t hear from gasoline marketers but should be hearing from the government: it takes more oil to manufacture premium gasoline than regular. The explanation requires a short discussion of organic chemistry. (Sorry.)

Crude oil consists of vast numbers of little chains of carbon and hydrogen atoms. These chains come in various lengths, measured by the number of carbon atoms included. The molecules in the gasoline fraction of crude oil generally have between 5 and 9 carbon atoms, with varying numbers of hydrogen atoms. Some of these molecules are “straight”, with each carbon atom connected to only one or two other carbons. Others are “branched”, with some carbon atoms connected to three or four other carbons, and correspondingly fewer hydrogens. Some even come in rings, with five or six carbons in a chain that connects to itself and creates a loop.

The reason this matters is that the branched and ring molecules have higher octanes than the straight molecules, which are more prevalent in most crude oil. (The reference standard for 100 octane is a twisty little guy called “2-2-4 trimethyl pentane.) Much of a modern refinery is devoted to changing the proportions of these different molecules in gasoline, and therein lies the problem. The commonest process for turning low-octane straight hydrocarbon molecules into high-octane rings is called catalytic reforming, or “Platforming.” (Remember the old Shell ads: “Super Shell with Platformate!) Unfortunately, in the reforming process up to 10-15% of the “naphtha”, or raw gasoline, feed is converted into small, low-value molecules that can’t go into gasoline.

This brings us back to our concern about oil consumption. Because premium gasoline requires more “reformate” than regular gasoline, and reformate consumes more naphtha in its production, this means premium gasoline uses more crude oil, perhaps as much as 5% more than regular. As a result of all this, buying lower octane gasoline—if your car can tolerate it—not only saves money, it saves oil, too.