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Regular or Premium: What Kind of Gas Should I Put in My Car?

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author photo by Aaron Gold October 2016

Will premium gasoline make your car run better? Can regular gasoline damage your engine? Does midgrade give you better gas mileage? Motorists often ask these questions -- and come up with the wrong answers. This short article will help you decide what kind of fuel to put in your car.

The Short Answer

Use the cheapest fuel for which your car's engine is designed. You'll find the minimum octane rating in the owner's manual, and cars that require premium fuel will usually say so on or near the gas cap. If your car specifies premium fuel, use the good stuff. Otherwise, there's (usually) no reason to buy anything but regular 87-octane fuel. (See the section on buying fuel at high altitude for the exception.)

The Long Answer

Gasoline is rated by octane. Generally, regular fuel is 87 octane, premium is 91 or 93, and midgrade is (surprise, surprise) somewhere in the middle.

To understand what octane means, we first have to look at the concept of preignition. Engines compress a mixture of air and fuel and ignite them with a spark. Under certain conditions, the fuel-air mixture can ignite early, which makes a knocking sound not unlike a coffee percolator -- this is called preignition. Nowadays, most cars have knock sensors that prevent preignition, so you'll rarely hear it.

Octane measures the gasoline's resistance to preignition; the higher the octane, the less likely it is to detonate. Contrary to what some believe, high-octane gasoline has the same energy content as low-octane gasoline.

Why Do Some Cars Need High-Octane Fuel?

Some high-output engines use a higher compression ratio in order to produce more power. They compress the fuel-air mixture to a smaller size, which creates extra heat that can cause the fuel to preignite. These high-compression engines need high-octane fuel to ensure the gasoline doesn't ignite early. If your car has an engine that requires premium fuel, it will say so in the owner's manual, and there will usually be something written on or near the gas cap.

When to Use Regular Gas

If your car is designed for regular 87-octane gas, you're in luck: You can safely use the cheap stuff. Premium fuel won't make the engine run better or produce more power. There's no benefit to buying premium fuel, at least not to you or your car; the gas station owner and the oil companies benefit plenty.

When to Use Premium Gas

If your car says "premium fuel required" and the owner's manual specifies 91- or 93-octane, then you should always fuel up with the good stuff. Back in the day, using low-octane gasoline in a car that required premium would likely cause damage. In a modern car, the knock sensor can retune the engine on the fly to avoid preignition, but this will most likely reduce power and fuel economy, so it's a false economy, and engine damage is still possible.

When You Can Take Your Pick

If your car says "premium fuel recommended," you have a choice -- you can run regular or premium. These engines generally develop more power and get better fuel economy on higher-octane fuel, although the difference may be slight. However, you won't damage the engine (or void your warranty) by running regular. Our advice: Run a few tanks of both regular and premium, and calculate your fuel economy. Decide for yourself if the differences in power and fuel economy (if they're even noticeable) are worth the extra cost of premium fuel.

If Your Car Says RON 95 or RON 98

Some European cars specify RON 95 or RON 98 -- but don't worry, you don't have to look for 95- or 98-octane fuel. These are European octane numbers calculated using a different method than the one used in the United States. RON 95 is equivalent to 87-octane in the U.S., and RON 98 equals 94-octane; for the latter, 93-octane fuel should be sufficient.

Buying Gas at High Altitude

When traveling at high elevations, you're likely to see gas stations that sell lower-octane fuel -- for example, 85-octane regular. The old-school reasoning behind this is that thinner air effectively lowers the engine's compression ratio, so lower-octane fuel is OK. This may work for old cars with carburetors and minimal emissions systems -- and we're talking really old cars here, those built before 1975 -- but modern fuel-injected engines detect the lower oxygen content of the air and adjust themselves accordingly, so they'll be expecting fuel of the recommended octane. These engines can compensate for lower-octane gasoline, but only so much.

Our advice: When driving at high altitudes, go by the numbers. If your car requires 87-octane, use that, even if it means buying midgrade. If your car requires premium fuel, you may not be able to find the high-octane fuel you need. Plan ahead: Fuel up at low altitudes, and if you don't think you can make it back to lower altitudes without gassing up, buy the minimum quantity you need of the closest octane you can find. Try to add fuel while there's still some left in the tank; the higher-octane fuel will effectively dilute the lower-octane fuel, so the average octane in the tank will be closer to what the car needs. Fill the tank with high-octane fuel as soon as it's available.

Diesel and E85

If your car has a diesel engine, you must use diesel fuel. Diesel engines use a different process to ignite and burn fuel. They won't run on gasoline, and fueling up with gasoline can result in expensive repairs. Most diesel vehicles say "diesel fuel only" on or near the fuel-filler.

E85 is a blend of 85 percent ethanol and 15 percent gasoline. Some gasoline-powered vehicles (known as flex-fuel vehicles) can also run on E85; they usually have a bright-yellow fuel cap. A flex-fuel vehicle running on E85 pollutes less but will generally get around 30 percent lower fuel economy. Don't put E85 in your car unless you're absolutely sure that it's E85-capable. The car may run, but you'll likely void your warranty.

This image is a stock photo and is not an exact representation of any vehicle offered for sale. Advertised vehicles of this model may have styling, trim levels, colors and optional equipment that differ from the stock photo.
Regular or Premium: What Kind of Gas Should I Put in My Car? - Autotrader