Fuel Additives, Air Filters and Old Gas: Car Care Myths That Cost You Money

Every driver hears car advice from somewhere.

It comes from a parent, a coworker, a forum or the label on a bottle at the parts store.

Some of that advice is solid.

Some of it was true decades ago and no longer fits modern cars.

And some of it was never true at all, but it still sells products.

The good news is that many of these claims have been tested.

FuelEconomy.gov, the official fuel economy site of the US Department of Energy and the Environmental Protection Agency, keeps a list of common myths.

This guide walks through the ones that matter most for everyday car care.

Myth: fuel additives can boost your mileage

Shelves are full of bottles and gadgets that promise better gas mileage.

Some claim to clean the engine, and others claim to change how fuel burns.

FuelEconomy.gov reports that tests have shown these additives and devices do not improve fuel economy.

The only exception it names is a full conversion that meets all EPA certification standards.

The site also warns that some products may damage your engine or increase tailpipe emissions.

So the money spent on a miracle bottle usually buys nothing at the pump.

It may even buy a repair bill later.

If you want better mileage, the proven steps are much less exciting.

Keep the tires inflated, use the right oil and drive smoothly.

Myth: a new air filter saves gas

This one used to be true.

On older cars with carbureted engines, a clogged air filter could waste fuel.

Modern fuel-injected engines work differently.

They have onboard computers that adjust the mix of fuel and air automatically.

According to FuelEconomy.gov, replacing a clogged air filter on these engines does not improve fuel economy.

It can, however, improve acceleration.

That applies to gasoline cars built from the early 1980s onward and to diesel engines.

So an air filter still matters.

Replace it on the schedule in your owner's manual, or sooner if you drive on dusty roads.

Just do not expect the gas gauge to move more slowly afterward.

Myth: old cars always lose fuel economy

Many drivers assume a car simply burns more gas every year.

FuelEconomy.gov says a properly maintained vehicle keeps its efficiency for many years.

The site notes that cars 10 or even 15 years old see little drop in fuel economy when they are maintained.

Fuel economy actually tends to improve during the first few thousand miles of break-in.

If an older car suddenly starts using much more fuel, age is probably not the reason.

Something likely needs attention, such as low tire pressure or an engine problem.

A lit check engine light deserves a visit to a mechanic, even when the car feels normal.

Myth: premium gas makes any car more efficient

Premium gas costs more, so many drivers assume it must do more.

FuelEconomy.gov says most cars see no benefit from premium under normal conditions.

The exceptions are cars designed for premium fuel and cars that knock severely on regular.

Under heavy duty, such as towing in hot weather, higher octane might help performance and mileage by a few percent.

Your owner's manual tells you whether premium is required, recommended or unnecessary.

If the manual says regular, premium is usually money you do not need to spend.

Myth: gasoline goes bad in a few months

Many people add a stabilizer to every gas can and every car that sits for a while.

A study sponsored by the Department of Energy looked at this question.

It found that gasoline stored in a sealed container at typical outdoor temperatures stayed within US gasoline standards for at least a year.

FuelEconomy.gov says stabilizers can improve stability, but they are not necessary for storage under 12 months in typical conditions.

That is useful for plug-in hybrid owners, who may go months without using a full tank.

The key words are sealed and typical.

A container left open or stored in extreme heat is a different situation.

Myth: restarting the engine uses more fuel than idling

This belief keeps many cars running in parking lots and pickup lines.

FuelEconomy.gov says modern fuel-injected engines start very efficiently, especially when warm.

Idling can use a quarter to a half gallon of fuel per hour, depending on engine size.

The site estimates that idling costs about 1 to 2 cents per minute.

Its advice is to turn off the engine when the car is sitting still.

The exception is traffic or a slow line, where you would start and stop too often.

Myth: the car needs a long warm-up before driving

Modern vehicles can be driven within seconds of being started.

FuelEconomy.gov adds that the engine should not face extreme loads until it reaches normal temperature.

The quickest way to warm up an engine is to drive it gently.

In cold weather, the site says most manufacturers recommend driving off gently after about 30 seconds.

Long warm-ups mostly burn fuel, since idling gets zero miles per gallon.

Myth: the EPA rating is a promise

The mileage on the window sticker is not a guarantee.

FuelEconomy.gov explains that the ratings exist to compare vehicles in a fair and uniform way.

No single test can model every driver, road and climate.

Fuel blends also matter.

The site notes that gasoline with 10% ethanol can lower fuel economy by about 3%.

So if your real mileage is a little lower than the sticker, that alone does not mean something is wrong.

A sudden drop compared with your own normal mileage is the better warning sign.

What actually helps

Once the myths are set aside, the useful habits are simple and cheap.

  • Check tire pressure monthly. FuelEconomy.gov says proper inflation improves mileage by 0.6% on average and up to 3% in some cases.
  • Use the right pressure. Find it on the sticker in the driver's door jamb or in the manual, not on the tire sidewall.
  • Use the recommended oil grade. The site says this can improve mileage by 1% to 2%.
  • Fix engine problems promptly. A check engine light can point to issues that raise fuel use and repair costs.
  • Drive smoothly. Gentle starts and steady speeds save more fuel than any bottle on the shelf.

Under-inflated tires deserve special attention.

FuelEconomy.gov says mileage drops about 0.2% for every 1 psi drop in the average pressure of all tires.

Properly inflated tires are also safer and last longer.

How to judge the next claim you hear

New products and tips will keep appearing.

A few questions help you sort them quickly.

  • Does the claim come from a test, or only from a label or a story?
  • Does it fit how modern fuel-injected engines work?
  • Does your owner's manual say anything about it?
  • Is the person giving the advice also selling the product?

When in doubt, the owner's manual and official test data are better guides than a bold promise.

Good car care is mostly about doing the basics on time.

That approach is not flashy, but it is what keeps a car running well and spending less on fuel.

Sources and Further Reading