Is it OK to use the lowest-grade gas? Here’s a guide to the different types of fuel.

What is octane rating?

The octane rating on different grades of gasoline is a measurement of the fuel’s stability. Higher octane means greater stability. The greater stability indicates the gasoline’s resistance to preignition and the fuel’s ability to resist “knocking” or “pinging” during combustion. These sounds come from the air-fuel mixture detonating prematurely in the engine.

Internal-combustion engines compress a mixture of air and fuel and ignite them with a spark. Under certain conditions, the fuel-air mixture can ignite early, creating a knocking sound in the engine. Newer cars have sensors that prevent preignition, so you’ll rarely hear knocks and pings unless you drive an older vehicle.

A gasoline’s octane rating averages two testing methods: the research octane number (RON) under idle conditions, and the motor octane number (MON) under higher engine speed. You might have noticed this formula in smaller letters on the yellow buttons displayed on gas pumps: (R+M)/2.

The larger numbers on gas-pump stickers indicate the octane rating of the different gasoline grades. Typically, gas stations sell gasoline with three octane levels.

Regular

Regular gas is the lowest-octane fuel at typically an 87 level.

Midgrade

Midgrade is the middle-range fuel at usually an 89 octane level.

Premium

This is the highest-octane fuel you can buy, and it’s generally at levels of 91 to 94.

Even though some companies market their different grades of gas with names such as “Super Premium,” “Ultra” or “Ultimate,” they all refer to the gasoline octane rating.

What gasoline or fuel should I put in my car?

The fundamental rule of car maintenance is to follow the manufacturer’s guidelines found in the owner’s manual. It will show the minimum octane level for the vehicle and whether premium gas is recommended or required. There’s a distinct difference.

Recommended vs. required fuel types

Cars that require premium fuel often have a warning notice at the gas cap and sometimes on the dashboard gas gauge reminding drivers to use the correct type of gas.

When your owner’s manual recommends a fuel type, it’s not required that you put it in your vehicle.

Generally, do what the manufacturer outlines in the owner’s manual and pay attention when filling up at the gas pump and make sure to choose the right fuel for your car.

Higher-octane fuel may be necessary for engines that use a greater compression ratio or use turbocharging to force more air into the cylinders. This allows high-output engines to produce more power from the compressed air-fuel mixture. These high-compression engines can require high-octane fuel to ensure the mixture doesn’t ignite early.

Using fuel with an octane level lower than what is recommended by the manufacturer can reduce engine performance. Over time, not following the octane guidelines for your car can cause damage to the engine and its emissions-control system.

It might also void the vehicle’s warranty.

Can you use a higher-octane gas?

If your car is designed for regular-octane gas, you can safely use higher grades of gasoline. But under normal driving conditions, there is little to no benefit to buying fuel with octane levels greater than what is recommended. However, if you have an older car experiencing engine knock, filling up with premium gas a few times might fix that problem.

If you unintentionally fill up your tank with a gasoline grade with an octane rating less than or greater than what is recommended for the car, there is no reason to panic.

What is a cause for concern is putting diesel fuel into the tank of a gas vehicle or pumping gasoline when the car requires diesel.

It’s difficult and uncommon, but not impossible, to put diesel fuel into a tank intended for gasoline. A diesel pump is a different color, usually green, and its nozzle is larger so that it won’t fit into a gasoline tank’s opening. If you accidentally put diesel into your gasoline tank, you will need to pay to have the engine’s fuel lines drained and cleaned at a dealership or local car-repair business. Similarly, having gasoline in a diesel engine can cause damage to the fuel pump, filter and injectors.

What is flex-fuel?

E85 is a fuel designed for “flex-fuel vehicles.” It is composed of 85% ethanol and 15% gasoline. E85 pumps are clearly labeled at gas stations and typically have yellow nozzles. Flex-fuel vehicles usually have a yellow fuel cap. Using gasoline of any octane level in a flex-fuel vehicle is OK. However, putting E85 in a car that isn’t designed for it can be harmful.

Most insurance policies don’t cover accidentally fueling up with the wrong gasoline type. However, some insurers offer riders that do cover the mishap. So check with your insurer and your policy. It is important to pay attention at the pump, avoid distraction and know your car’s requirements.

Understanding diesel fuel

Diesel fuel is the common term for petroleum distillate fuel. Gasoline and diesel engines operate differently and therefore use different types of fuel. Diesel is used in vehicles that use compression-ignition engines, while gas engines are spark-ignited. In general, diesel engines work more efficiently and provide greater torque that is needed to move heavy loads, even though they generate less horsepower than their gasoline counterparts.

These days, the diesel car market in the U.S. is relatively small. However, many heavy-duty pickups and vans are offered with optional turbo-diesel engines that bring the towing capacity that is desired for commercial work and off-road driving or for pulling boats and campers.

Not every diesel-powered vehicle looks like a workhorse, however. The Land Rover Range Rover, Land Rover Range Rover Sport, Cadillac Escalade and Jeep Gladiator offer diesel powertrains.

In addition to petroleum-based fuel, diesel engines are capable of using biodiesel that is produced from raw vegetable oil, used cooking oils and waste animal fats.

What is recreational fuel?

Ethanol-free 90-octane gasoline is sometimes marketed as “recreational fuel” or REC-90. Many consumers prefer to use this pure gasoline that isn’t blended with ethanol for marine equipment and small engines for lawnmowers, snowblowers, chainsaws, generators, pumps, and the like.

Non-ethanol gasoline typically costs more than varieties that are blended with ethanol. This type of gas isn’t sold at every filling station.

Using ethanol-free gas in your car won’t harm the vehicle’s engine. However, using an ethanol blend may cause damage to your outdoor power equipment. Refer to your owner’s manual for the correct fuel to use for your tools.

Why do some gasoline types cost more than others?

The most noticeable difference between grades of gasoline is the cost, and motorists often wonder why higher-grade options cost more. The reason is that the additives and components used for boosting octane are generally more expensive to produce.

As of March 17, the average price of regular unleaded in the U.S. was $4.29 per gallon, while a gallon of mid-grade costs $4.67, according to AAA. Drivers buying high-octane premium gasoline can expect a price increase of 66 cents more than regular, or $4.95 per gallon on average, the website shows.

That means it costs $9.90 more to fill a 15-gallon tank with high-octane premium gas than it would with lower-octane regular.

Is gas with higher octane worth the price difference?

Absolutely. However, it’s only worth the price difference if your car requires higher octane gasoline such as mid-grade or premium fuel. Otherwise, you would be wasting money.

Some owner’s manuals indicate that using fuel with a higher octane rating will make your car run better or more efficiently. Still, the increase in cost per gallon is typically higher than any reduction in gas consumption.

Price factors for gasoline

Many factors determine gasoline prices. Drivers often see higher prices in the summer driving season. Supply from refineries and demand from vacationing motorists can add up to 15 cents per gallon to gas prices in the summer months. Additionally, seasonal gasoline blends found at the pump contribute to price differences from one season to the next.

In warmer months, gasoline has a greater chance of evaporating from a car’s fuel system. To help offset smog and emissions, refiners produce a summer blend with lower volatility to reduce the chance of evaporation. Blends vary by state because of government regulations. These seasonal blends can add up to 15 cents per gallon on top of demand increases, depending on the region.

From September to April, cooler weather brings lower gas prices because the winter blend is cheaper to produce. The price at the pump can fall as much as 30 cents per gallon as gas stations switch to winter gasoline and many drivers stay closer to home.

This story originally ran on KBB.com.