GREENSBORO, N.C. (WGHP) — You may have seen the reports that Sheetz is dropping the price of gas to $3.99, but the deal isn’t for your usual “regular,” “plus” or “super” grades of gas.

A release sent out Monday by Sheetz said all of its stations would cut the price of unleaded 88 gas to $3.99, and ethanol 85 flex-fuel to $3.49, effective immediately. For comparison, the average unleaded price in North Carolina was $4.505 as of Tuesday afternoon.

But keep in mind that unleaded 88 and E85 flex fuel are two very specific grades of gas, and it may not be compatible with your vehicle.

Sheetz didn’t mention a change in price for its other fuel types—often labeled as “regular,” “plus” and “super” at Sheetz locations—so you’re likely to continue seeing those prices well over $4 a gallon.

What do the numbers on the gas pump mean?

The numbers stand for the octane rating of the gas. The U.S. Department of Energy said the rating can tell how well the fuel will resist engine knocking, which is when fuel burns unevenly in the engine and potentially causes damage to the engine’s cylinder and pistons, according to Firestone Auto Care.

Most cars can use unleaded 87, but some luxury cars require higher-grade gas types to work with their high-performance engines.

The U.S. Department of Energy breaks octane ratings down as follows:

  • Unleaded 87 – “Regular” grade gas
  • Unleaded 88 through 90 – “Midgrade”
  • Unleaded 91 through 94 – “Premium”

Can I use gas of a higher/lower octane rating than required for my vehicle?

You can generally use gas of a higher octane rating than required for your car, but you should not use gas of a lower octane rating.

Using gas with a lower octane rating than required could make your vehicle run poorly or damage the engine and emissions control system over time. For example, if you have to use regular Unleaded 87, don’t reach for the E85 flex-fuel.

“In older vehicles, the engine can make an audible ‘knocking’ or ‘pinging’ sound,” the Department of Energy said of using a lower octane fuel than required. “Many newer vehicles can adjust the spark timing to reduce knock, but engine power and fuel economy will still suffer.”

That said, gas with a higher octane rating than necessary could actually help your vehicle in some circumstances. For example, if you usually use regular gas, you could see some potential benefits using premium.

“For most vehicles, higher octane fuel may improve performance and gas mileage and reduce carbon dioxide (CO2) emissions by a few percent during severe duty operation, such as towing a trailer or carrying heavy loads, especially in hot weather,” the Department of Energy said in its explanation of octane ratings. “However, under normal driving conditions, you may get little to no benefit.”

Can my car use unleaded 88?

Sheetz specifically said the following kinds of cars can use its unleaded 88 gas:

  • Cars made in 2001 or later
  • Light-duty trucks
  • SUVs
  • Flex fuel cars

The best way to know what kind of gas to use in a car, according to the U.S. Department of Energy, is to check the car’s owner’s manual to see what octane rating the manufacturer recommends using. However, the government agency also said while gas rated lower than the manual requires can damage a car’s engine, it’s possible to use gas at a higher octane rating than what the manual says.

Can my car use E85 flex fuel?

E85 flex fuel, the other gas at a discount with Sheetz, is specifically for cars labeled as being able to use flex-fuel. The U.S. Department of Energy said a common way to tell if a car is flex fuel-approved on models after 2008 is to see if it has a yellow gas cap, or an “FFV” or “Flex Fuel” badge on the car’s body.

However, the government agency also said a flex fuel car using E85 instead of unleaded 88 also gets anywhere from 15 to 27% fewer miles per gallon.

Sheetz said its dramatically lower price points for unleaded 88 and E85 are intended to reduce pain at the pump for Americans, and that the offer would be valid through the July 4 holiday travel season.

View the map below to find a nearby Sheetz location.