The short answer
Costco keeps it simple — most fuel centers sell just three fuel types: regular (usually 87 octane), premium (usually 91), and diesel at select warehouses. There's no 89 mid-grade. The only difference between regular and premium is the octane number — both carry the same Top Tier detergent. Octane measures resistance to engine knock, not quality or power, so the right grade is whatever your owner's manual asks for. For most cars that's regular; paying for premium you don't need is money down the drain.
Pull up to a Costco fuel center and the choice looks reassuringly short: two gasoline buttons, maybe a diesel pump off to the side. But what actually separates them — and which one your car should be drinking — trips up a lot of drivers. Here's exactly what each Costco fuel type is, what the numbers mean, and how to stop overpaying at the pump.
The three fuel types at Costco
Costco offers regular and premium gasoline, plus diesel at select locations. That's the whole menu. Notably, you'll never find 89-octane mid-grade at a Costco — unlike most branded stations that post three gasoline grades, Costco stocks only two.
| Fuel type | Octane (U.S. AKI) | Best for | At every Costco? |
|---|---|---|---|
| Regular | 87 (85–86 at altitude) | The vast majority of cars | Yes |
| Premium | 91 (93 in some states) | Engines that require or recommend premium | Yes |
| Diesel | — (rated by cetane, not octane) | Diesel engines only | Select locations |
What "octane" actually means
The big number on the pump — 87, 91, 93 — is the octane rating, and it's the single most misunderstood thing about gasoline. It is a measure of a fuel's ability to resist "knocking" or "pinging" — the rattling sound an engine makes when the fuel-air mixture detonates too early. Higher octane simply resists that premature ignition better. It is not a measure of how much energy, power, or cleaning power the fuel has.
In the United States the posted number is the Anti-Knock Index (AKI), written on the pump as (R+M)/2— the average of two lab tests (Research and Motor octane). That's why American octane numbers (87/91/93) look lower than the 95/98 you see in Europe, even for comparable fuel: it's a different scale, not weaker gas. As AAA puts it bluntly, premium is "higher octane, not higher quality".
Regular (87 octane) — what most cars should use
Regular is Costco's standard unleaded, typically rated 87 octane. The U.S. Department of Energy notes that most gasoline vehicles are designed to run on 87 octane, so for the average sedan, SUV, or minivan, regular is exactly the right fuel — not a compromise. Like nearly all U.S. gasoline, it's E10 (up to 10% ethanol), which federal renewable-fuel law requires in the supply and which is labeled on every pump.
Premium (91, sometimes 93) — and who really needs it
Costco's premium is usually 91 octane, though in some states the top grade is 93. The only physical difference from regular is the octane rating — the detergent package is identical. So who should pay for it? Two groups:
- "Premium required." If your owner's manual or fuel-filler door says premium is required, use it. These are typically turbocharged or high-compression engines calibrated for high octane, and the DOE warns that feeding them lower octane than required means the engine "can run poorly and can damage the engine and emissions control system over time" — and may even void your warranty.
- "Premium recommended." If the manual says premium is merely recommended, it's your call. The car will run fine on regular; you may give up a sliver of performance. The DOE notes that here the cost increase is typically higher than the fuel savings.
For everyone else, premium is money wasted. A landmark 2016 AAA study tested 87- and 93-octane fuel in vehicles built for regular and found no benefit — no extra horsepower, no better fuel economy, no cleaner emissions. AAA estimated U.S. drivers burned $2.1 billion a yearon premium their cars couldn't use. Federal regulators agree: the FTC says using higher octane than your manual recommends offers "absolutely no benefit" and that "unless your engine is knocking, buying higher octane gasoline is a waste of money." The one real-world exception the DOE flags: a higher grade may slightly improve performance when towing or hauling heavy loads in hot weather.
Why there's no mid-grade (89) at Costco
At most stations, the middle button is 89-octane mid-grade — and at the pump it's literally a blend of the regular and premium tanks. Costco skips it. Very few vehicles specifically require 89, so offering only regular and premium keeps the pumps and the supply simpler. If your car calls for 89, you have two easy options at Costco: run 87 as long as the engine doesn't knock, or step up to 91 to stay on the safe side.
Diesel — a completely different fuel
Diesel isn't "another grade of gas" — it's a different fuel for a different kind of engine, and it's not interchangeable. Putting diesel in a gasoline car (or gasoline in a diesel) can cause serious, expensive damage, which is why diesel pumps use a larger nozzle. Diesel isn't rated by octane at all; its combustion quality is measured by cetane.
Costco sells Kirkland Signature Diesel — ultra-low-sulfur diesel (ULSD)that meets the federal cap of 15 parts per million sulfur, blended with deposit-control additives to keep injectors clean. It's also Top Tier certified. The catch: diesel is only at select warehouses, often those near highways or in industrial areas. Use the warehouse finder on Costco's website or the mobile app to filter for a diesel pump before you make the trip.
A note for high-altitude warehouses
If you fuel up at a Costco in the Mountain West, the numbers shift. In high-elevation states like Colorado, Utah, Wyoming, Idaho, Montana, and New Mexico, "regular" is often 85 or 86 octane, and 87 is sold as mid-grade. The thinner air at altitude lowers cylinder pressure, which historically reduced the octane an engine needed to avoid knock. The DOE confirms 85-octane gas is available in some high-elevation areas. Modern turbocharged engines don't benefit the same way, so if you drive a turbo, check your manual before grabbing the cheapest button.
What every Costco fuel type has in common
The grades differ in octane, but they share the things that matter for engine health. Every grade of Costco gasoline is Top Tier certified — an automaker-backed standard requiring far more cleaning detergent than the EPA minimum — and Costco says it doses about five times the EPA's required detergent level in both regular and premium. The base fuel comes from the same major refineries that supply other brands in your region. We covered that quality story in depth in Is Costco gas good? — the short version is that the cleaner-burning detergent comes standard on every grade, not just premium.
So which Costco fuel should you use?
Let your owner's manual — or the label inside the fuel door — decide. If it says regular, use regular and pocket the difference; the premium pump won't make a regular-spec car faster or cleaner. If it says premium is required, use premium. If you drive a diesel, find a Costco that has a diesel pump. The grade is the easy part — the bigger win is that Costco's pump price usually runs below nearby stations, which you can check on our live price pages.
Frequently asked questions
How many types of gas does Costco have?
Two grades of gasoline — regular (usually 87 octane) and premium (usually 91 octane) — plus diesel at select warehouses. Costco does not sell 89-octane mid-grade, so most fuel centers have exactly two gasoline buttons at the pump.
What octane is Costco premium gas?
Usually 91 octane (the (R+M)/2 number posted on the pump). In some states — parts of the East and Midwest — the top grade is 93 instead, because Costco follows the regional norm. The only difference between Costco regular and premium is the octane rating; both get the same Top Tier detergent.
Is Costco premium gas worth it?
Only if your owner’s manual says premium is required or recommended — typically turbocharged or high-compression engines. For a car built for regular, a 2016 AAA study found premium delivered no extra horsepower, fuel economy, or lower emissions, and federal regulators say it offers “absolutely no benefit.” You would just be paying more.
Does Costco sell diesel?
At select locations, not all of them. Costco’s diesel is Kirkland Signature ultra-low-sulfur diesel (ULSD) and is Top Tier certified. Use the “Find a Warehouse” tool on Costco’s website or the mobile app to filter for stations that have a diesel pump before you drive over.
Why doesn’t Costco have 89 mid-grade gas?
Costco deliberately stocks only regular and premium. Mid-grade (89 octane) is essentially a blend of the two, and very few vehicles specifically require it, so skipping it simplifies the pumps and the supply chain. If your car calls for 89, you can run 87 if it doesn’t knock, or 91 to be safe.
Can I put premium in a car that takes regular?
You can — it won’t damage the engine — but for most cars it’s a waste of money. The U.S. Department of Energy says using higher octane than your vehicle requires gives “little to no benefit” under normal driving. The main exception is slightly better performance when towing or hauling heavy loads in hot weather.
Octane ratings, grade availability, and store policies vary by region and change over time — figures were last reviewed in June 2026. Always check the grade and price at the pump, and follow your vehicle's owner's manual. This site is independent and not affiliated with Costco Wholesale Corporation.
