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The Allstate Blog | Everyday Peace of Mind

Putting Conventional Car Wisdom on Trial

February 20, 2019 Many of us grow up acquiring car knowledge from multiple sources of varying authority. Your dad. Your uncle Jim. A local radio host. Your mom. Facebook. Most of the time, you probably didn't have an expert around to confirm or debunk the conventional wisdom you were receiving. Fear not! Fortunately,… Allstate
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Many of us grow up acquiring car knowledge from multiple sources of varying authority. Your dad. Your uncle Jim. A local radio host. Your mom. Facebook. Most of the time, you probably didn’t have an expert around to confirm or debunk the conventional wisdom you were receiving.

Fear not! Fortunately, we reached out to Dr. Christopher Depcik, an associate professor of mechanical engineering at the University of Kansas, and volunteered six bits of conventional wisdom for him to evaluate. His responses may surprise you:

Conventional Wisdom: Top off your tank when possible.

Verdict: Wrong! Topping off your gas tank isn’t a good idea. 

Refueling car with pump nozzle“There are a number of reasons why it is not a good idea,” Depcik said. “Probably the two that are of the most interest are: Any fuel that you try to add might feed back to the gas station because your tank is full; hence, you are paying for nothing. It increases the spill hazard that can be quite detrimental to your health and the environment.”

According to Angie’, topping off your gas tank can interfere with a gas station’s vapor recovery system, potentially allowing pollutants into the air. Gas also releases vapors containing irritants that can affect your breathing and, if spilled on the ground, could evaporate and further contribute to air pollution.

Conventional Wisdom: Your smartphone can start a gas fire at the pump.

Verdict: There’s no evidence that common use of a smartphone can ignite gasoline

“Yes, gasoline is highly volatile and when you’re filling your gas tank, it could potentially reach high enough concentrations to start a fire, if given an ignition source,” Depcik said.

Yet not every stimulus is an ignition source. While the Federal Communications Commission (FCC), is aware of reports and rumors of gas station fires resulting from cell phone usage, the FCC says that there is no evidence to support claims that wireless devices such as smartphones can trigger pump-related fires. 

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Conventional Wisdom: Premium gasoline is always better.

Verdict: Wrong! There are differences between regular and premium gas, and it’s important to fill your car with the recommended type of fuel.

In the past, drivers used to fill up with premium every now and then to clean out their engine, according to That’s because premium gasoline used to contain additives and detergents to reduce carbon deposits. Today’s gasoline–all grades–contains additives to help limit pollution and reduce engine wear, says Edmunds.

Super gas — also known as premium — has a higher octane number than regular gas, according to Depcik.

“There are a few differences between them, including certain additives that are included in the super gas that can lead to a reduction in deposits in the engine. To me, the biggest difference is the higher octane number,” he stressed.

So, what’s the takeaway? According to Ahmed Ghoniem, a mechanical engineering professor at the Massachusetts Institute of Technology, filling a tank with a higher-octane gas improved efficiency for engines designed to run on that specific kind of fuel. “One can argue that using high-octane fuels in the right engine (one designed for higher-octane fuel; specified in your vehicle’s owner’s manual) ultimately leads to more mechanical power from the same amount of fuel,” Ghoniem said in response to a question submitted to MIT’s “Ask An Engineer” series.

Still, if your car’s manufacturer doesn’t specify premium and your engine runs fine on regular gasoline, then spending more money on a higher-octane gas isn’t going to benefit your car or your wallet, William Green, a chemist at MIT, told Scientific American.

Conventional Wisdom: Gently break in a new car.

Verdict: Wrong! There’s a right way, and a wrong way, to break in a new car—and what you think you know about the process might be outdated.

Popular Mechanics reports that while many people have opinions on breaking in a new car, they’re not always right. In fact, “The technology of building a modern automobile has evolved to the point where a lot of ‘wisdom’ is obsolete,” writes Popular Mechanics’ Mike Allen.

Depcik explains that not everyone knows the proper way to break in a new car. “Typically, the first little bit of break-in has been performed by the dealer or place of manufacture, along with an oil change,” he said.

If you’ve heard that you shouldn’t drive faster than a certain speed for the first 1,000 miles or so, you might also want to reconsider that approach, according to Depcik. “Personally, I believe that stop-and-go driving around town … is a nice way of getting the engine set up to handle the long haul,” he said. “Going for a long trip right away may not stress the engine the way it needs to be stressed.”

According to Popular Mechanics, though start-and-stop driving is unlikely to harm your new car, you should avoid high speeds and refrain from towing heavy trailers for the first 1,000 miles. What’s more, thanks to advances in automotive paint technology, the old adage about refraining from applying wax to a new car for 90 days is also outdated.

Conventional Wisdom: Let your car warm up on cold days

Verdict: Wrong! Despite what you might have heard, you don’t need to warm up your car before driving it on very cold days.

If you grew up in an area with cold winters, then there’s a good chance you were told that your car needed to “warm up” before being driven. As it turns out, that advice may not be so sound. “Contrary to popular belief, idling isn’t an effective way to warm up most car engines,” says the Environmental Protection Agency (EPA). “Today’s automobile manufacturers recommend driving off right away and urge that drivers wait no more than 30 seconds to begin driving, even on the coldest days.”

Indeed, idling a vehicle also releases environmental pollutants, reports the EPA, and it doesn’t accomplish its intended goal of warming up the car. “The engine is not producing much power or heat when idling; hence, there is not much energy available for heating the cabin, and the oil warms relatively slowly,” Depcik said. similarly advises against idling vehicles in cold weather: “Most manufacturers recommend driving off gently after about 30 seconds. The engine will warm up faster being driven, which will allow the heat to turn on sooner, decrease your fuel costs, and reduce emissions.”

Conventional Wisdom: Don’t let your gas level fall too low

Verdict: You’re right! Depending on the type of engine, it could be detrimental to let your gas tank level fall too low.

If your parents warned you about letting your gas tank level fall too low, then they might have been on to something. “With the newer injection technologies, you do not want to run out of fuel because that can allow air in the lines and make it quite difficult to restart the engine,” Depcik said.

According to Consumer Reports, habitually waiting until your gas tank is nearly empty before refueling could also potentially have a negative impact on non-diesel engines. “While the cases are rare, there is real potential of a costly mechanical problem,” explains John Ibbotson, a supervisor at the Consumer Reports Auto Test Center Shop, on the company’s website.

That’s because gasoline functions like a coolant for electric fuel-pump motors. If levels are too low, the pump could instead draw in air and possibly damage itself. Additionally, dirt in the gas tank could also block a car’s fuel filter, necessitating a costly repair. Consumer Reports advises not to let the gas tank go below one-fourth full.

Improving upon your conventional car wisdom can help you increase your awareness of car maintenance and also keep you and your passengers safe.

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