Fuel FAQs: Everything you need to know about filling up

By Allstate

Last updated: January 1

Find the answers to some common questions that may arise when you pull up to the pump.

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Is your gas cap on the left or right?

The answer can often be found in a small symbol on your dashboard. Look for an arrow or triangle near the gas gauge. This indicates which side of the vehicle the gas tank is on. If the arrow points left, the car's fuel door is on the left. If it points right, it's on your right. This trick will make it easier to ensure you pull in on the correct side of the gas pump.

Arrow indicators on new cars

The gas tank arrow is on most modern cars, including most rental cars, according to AutoGuide.com.

Gas pump icon on old cars

AutoGuide.com says there is no consistent gas cap indicator on older cars. Sometimes the hose on the gas pump indicator is on the same side of the car as the gas cap, but this is not always the case.

Why aren't all fuel doors on the same side?

The United States doesn't currently have any official rules that dictate which side of a car the fuel door should be placed on. That means the decision is left in the hands of the vehicle manufacturer. According to Jalopnik, there's no real consensus in the industry, either. Some car companies choose the left side, or driver's side, based on convenience for the driver. Others consider the passenger's side safer for drivers when they need to fill up (say, on the side of the road). Manufacturers also take into account the placement of the car's hardware and undercarriage.

In a column for PBS News Hour, economist Robert Frank proposed that some cars having their fuel doors on the right and some on the left results in shorter lines at the gas pump, since it means both sides of the fuel pump can be used more easily. This prevents every driver from needing to wait for the same pumps.

What happens when you use the wrong kind of fuel?

Getting fuel types mixed up at the pump could result in major engine problems. But some mix-ups cause more damage than others, so here are some specifics on how the wrong fuel can affect certain engines.

What happens when gasoline goes into a diesel engine?

Putting gasoline into your diesel engine could produce catastrophic results for your engine and its parts, according to J.D. Power. If you mistakenly fill your diesel engine with gasoline, do not start the engine. Your vehicle will need to be towed to a repair shop or facility who can drain the tank and advise of next steps.

This is because diesel engines ignite fuel using the heat created when the fuel is compressed by the engine piston, according to the U.S. Department of Energy (DOE). This is unlike gasoline engines, which use spark plugs to ignite the fuel.

Compression ratios for diesel engines are typically 16:1 or higher. On the other hand, gasoline engines usually have a compression ratio of 9:1 or 10:1, according to Engine Builder Magazine. This difference in compression ratios causes gasoline to have higher combustion tendencies than diesel.

What if you put diesel into a gasoline engine?

Mistakenly filling up a gas-fueled car with diesel may not ruin your engine, but it could ruin other parts of your fuel system, like your fuel injectors or your fuel pump, according to J.D. Power. That's because diesel is thicker and denser than gasoline, which makes it difficult for the fuel pump to move it through the system. Eventually, any diesel fuel that makes its way to the engine will quickly clog up the fuel injectors.

Rather than running the engine, have your car towed to a professional who can ensure the fuel system is drained and cleaned.

What if I put E85 in a vehicle not made for it?

According to the Department of Energy, most gasoline in the U.S. contains ethanol. Traditional unleaded fuel typically contains a ratio of 10 percent ethanol to 90 percent gasoline. However, flex-fuel vehicles are designed to run on E85, which has a ratio of roughly 85 percent ethanol to 15 percent gasoline.

If your car isn't a flex-fuel vehicle and you mistakenly add E85 to your tank, you may notice reduced performance and gas mileage. Your check engine light may also appear, but the mishap likely won't damage the engine. They recommend topping the tank off with regular gasoline several times. When all the E85 has cycled through the engine, the check engine light should turn off.

What should I do if I put the wrong fuel in my car?

According to NAPA Auto Parts, here are the steps you should take if you fill up with the wrong type of gas.

If you notice the error while you're still filling up

Stop the pump right away. Do not start the engine or even turn the key to the on position, as it'll begin pumping the wrong fuel through the system. Push the car away from the pump to a safe place, then call for a tow truck to take your car to a repair shop where they can drain and clean the tank.

If you don't notice the error immediately

If you realize you added the wrong fuel after you have started driving, pull over as soon as it's safe and shut off the engine. Have the vehicle towed to a repair facility where they can determine the steps necessary to drain, flush and repair the fuel system.

How can I avoid putting the wrong fuel in my car?

Gas pumps have been designed to help you avoid filling up with the wrong fuel. Diesel nozzles are wider than gasoline nozzles, which means a diesel nozzle won't fit into your gasoline vehicle's tank. Diesel drivers should take extra caution, though, because the smaller gasoline nozzle will easily fit into diesel filler necks.

Some auto manufacturers are coming up with new devices that can be installed to prevent diesel drivers from accidentally filling up with gasoline, but if your vehicle doesn't have one of these, take extra caution at the pump.

Pump handles are typically color coded. But note that the colors aren't always consistent from gas station to gas station. That means the best way to avoid costly gas errors is to always take extra caution and read the pump when you pull in.

What's the difference between premium and regular unleaded gasoline?

Knowing octane ratings is key to understanding the difference between premium and unleaded gasoline.

According to the DOE, fuel contains oxygen. This oxygen helps keep gas from prematurely igniting in the engine (known as "knocking"). An octane grade or rating is based on how much oxygen is in the fuel.

What is unleaded gasoline?

Unleaded gasoline is fuel with an octane rating of around 87. However, the DOE says there are three different octane ratings for fuel:

  • Unleaded Fuel: 87
  • Mid-grade Fuel: 88-90
  • Premium Fuel: 91-94

Yellow stickers on gas pumps are required by the Federal Trade Commission (FTC) to indicate octane levels and help ensure consumers have the information they need.

How is regular unleaded gasoline different from premium?

Premium fuel has a higher octane level compared to regular unleaded or mid-grade fuel. According to the FTC, higher octane ratings make fuel more resistant to "knocking."

Knocking is caused when fuel prematurely combusts in one or more cylinders in the engine, says the FTC. This can lead to an audible engine knocking or pinging sound when you start your car. Consumer Reports notes that knocking could result in damage to the car's engine over time.

The majority of cars take regular unleaded fuel, says Consumer Reports. Using plus- or premium-grade fuels typically doesn't affect your engine's performance or resistance to wear-and-tear.

Check your vehicle's owner manual or ask a dealership for help to find out what kind of fuel is recommended for your car.

What factors determine gas prices?

Here are a few factors that affect what you pay at the pump:

Price of crude oil

The biggest contributor to the price of gas is the cost of crude oil, says the U.S. Energy Information Administration (EIA). Also known as fossil fuel, crude oil is the liquid that is removed from the ground and sent to refineries to be made into gasoline.

Crude oil prices are based largely upon supply and demand. So, when crude oil production rises, prices may decline. Conversely, costs may rise when production declines. Economic conditions play a role in crude oil prices, the EIA says. When the economy is doing well, demand for raw materials, such as crude oil, tends to rise and prices tend to fall.


State and federal taxes are built into the price of a gallon of gas, says Energy.gov. While the same federal tax rate is charged at every gas station in the U.S., each state sets its own tax rates. Some states charge sales tax, while others may include other costs, such as environmental fees, in the price of a gallon of gas.


Gas prices can vary based on state or region due to the distance from the supply, according to the EIA. Supply sources can include the pipeline and refineries. Say the source of the gasoline is near the Gulf Coast. States in that region may see lower prices at the pump because they are closer to the source than states along the West Coast or in New England.

Some states have adopted laws that require the use of "cleaner" fuel that has less environmental impact. This fuel typically costs more to produce and is offered by limited suppliers, making it more expensive for drivers to purchase, the EIA explains.

Gas station owners may adjust their prices based on how many competitors are nearby, says the EIA.

Additional factors

When setting their prices, gas station owners may also consider their operating costs, like employee wages and rent payments, the EIA says. Other expenses, such as fuel distribution, marketing and refining, also play roles in the price of gas.