Every gas station owner knows the four buttons or handles on a pump: regular, plus, super or premium, and diesel. Fewer people can say precisely what separates them. This guide explains the numbers in plain terms, using public figures from the US Energy Information Administration (EIA) and the US Environmental Protection Agency (EPA). It is general information about fuel sold in the United States and says nothing about the specific products or prices of any supplier.
What octane actually measures
Octane rating describes how well a gasoline resists knocking, which is the uncontrolled, early ignition of fuel in an engine cylinder. A fuel with a higher octane rating tolerates more compression before it ignites by itself. That matters for engines built with higher compression or with turbochargers, whose makers specify a minimum octane rating.
Octane is not a measure of energy content, power or cleanliness. Putting a higher grade in an engine designed for regular does not, by itself, make the car faster or the fuel cleaner. It is simply a different resistance to knock.
Why the pump number is an average
Labs test octane in two ways. The research octane number, or RON, uses a lower engine speed and cooler air. The motor octane number, or MON, uses harder running conditions. The number posted on a pump in the United States is the average of the two, written (R+M)/2 and sometimes called the anti-knock index. Because other countries post RON alone, the same fuel can show a higher number abroad, which is why imported cars’ manuals can seem to ask for a higher figure.
The three gasoline grades
According to the EIA, the grades fall in these general ranges:
- Regular: generally 87.
- Midgrade, sold as plus: generally 89 to 90.
- Premium, sold as super or premium: generally 91 to 94.
Some regions, particularly at higher altitude, post lower numbers for regular because thinner air reduces knock. New Jersey is near sea level, and 87 is the common regular grade here. Exact figures depend on what a supplier delivers, so a station owner should confirm the posted rating of each grade with the supplier rather than assume it.
Many stations produce plus by blending regular and premium in the dispenser or the tank system. Whether a particular station does that, or stores a separate midgrade, depends on its equipment. If you are planning tank space, say how many tanks you have and what you sell from each when you ask a supplier about supply.
Diesel is a different system
Diesel engines ignite fuel by compression alone, so the octane scale does not apply. Instead of octane, diesel is described by measures such as cetane number, which relates to how readily it ignites, and by its cold-weather behavior. On-road diesel in the United States is ultra-low-sulfur diesel, or ULSD, and the EPA limits its sulfur content to 15 parts per million. That limit phased in from 2006 and applies to highway diesel today.
Cold weather matters for diesel more than for gasoline, because the wax in diesel can thicken in the cold. Suppliers commonly adjust the product for winter. If you sell diesel to trucks or fleets in New Jersey, it is reasonable to ask how winter blending is handled.
Why this matters when buying wholesale
When you compare suppliers, you are usually comparing the same four products, so the differences are in how they are delivered and sold. Knowing what the numbers mean helps you ask better questions, such as:
- What posted rating does each gasoline grade carry?
- Is the diesel ultra-low-sulfur, and how is it handled in winter?
- Are there additives or a branded program, and are they optional?
The fuel products page on this site places the grades on one scale. For the commercial side of the decision, see the guide to questions to ask a wholesale fuel supplier, and if you would like to start a conversation about your own station, use the request form.
Sources
The octane ranges and the (R+M)/2 explanation are from the EIA’s Energy Explained pages on gasoline and octane. The 15 parts per million sulfur limit is from the EPA’s diesel fuel standards pages. Check those sources for current figures, because regulations and typical ratings can change.