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The octane rating on different grades of gasoline is a measurement of the fuel’s stability. Higher octane numbers mean 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 gas is the lowest octane fuel at typically an 87 level.
Mid-grade is the middle-range fuel at usually an 89 octane level.
This is the highest-octane fuel you can buy and it’s generally at levels of 91-94.
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.
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 emissions control system.
It might also void the vehicle’s warranty.
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 with 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 for a few tanks 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. Similarly, having gasoline in a diesel engine can cause damage to the fuel pump, filter, and injectors.
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.
Fuel labeled as E15 is gasoline blended with up to 15% ethanol. About 2,500 stations in 30 states sell the fuel. It typically costs about 10 cents less per gallon than regular unleaded.
E15 isn’t usually available in warmer months because of concerns around higher ethanol content contributing to smog in hot weather. However, the U.S. Environmental Protection Agency issued an emergency waiver in April 2022, allowing summertime sales to combat higher prices at the pump. When it issued the waiver, the EPA said that drivers can use E15 without worrying that it will impact air quality.
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 is relatively small. However, many heavy-duty pickups and some 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.
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.
Gas stations market ethanol-free 90-octane gasoline 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 ethanol-blended varieties. 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.
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.
At the time of this writing, the average price of regular unleaded in the Thailand is ฿ 39.38 THB per litre, while a litre of mid-grade costs ฿ 39.65 THB, according to PTT Station. Drivers buying high-octane premium gasoline can expect a price increase of 67 cents more than regular, or ฿ 45.14 THB per litre on average, the website shows.
That means it costs more to fill a tank with high-octane premium gas than it would with lower-octane regular.
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.
Many factors determine gasoline prices. Drivers often see higher prices in the summer driving season in different countries. Supply from refineries and demand from vacationing motorists can add up to 15 cents per litres 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 in different countries. Blends vary by state because of government regulations like in the USA. These seasonal blends can add up to 15 cents per gallon on top of demand increases, depending on the region.
From cooler weather months brings lower gas prices because the winter blend is cheaper to produce in different countries. The price at the pump can fall as much as 30 cents per litre as gas stations switch to winter gasoline and many drivers stay closer to home.