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Is there a difference in the quality of gasolines?

Photo by Alamy Stock Photo

Wondering if you can buy the least expensive gasoline with no unfortunate consequences to your car’s engine? Sorry, but my own anecdotal evidence as well as some convincing research by AAA suggests that the answer is a definite no.

After the dreaded check engine light came on in my car several summers ago, I suffered a near-$500 blow to my budget for repairs, plus a scolding from my trusted mechanic because I consistently penny-pinched at the gas pump.

I’m not the only penurious motorist. A study by AAA found that American motorists are six times more likely to choose a gas station based on price rather than on the quality of the fuel.

Here’s the problem: The combustion process—gasoline and air burned in the cylinders to produce power—also produces byproducts, carbon being the most relevant to this discussion. Carbon deposits inside the engine disturb airflow and air/fuel ratios, adversely affecting performance, emissions, and fuel economy to the point that it might set off the check engine light—and trigger the need for potentially expensive repairs.

Decades ago, it was routine procedure to partially dismantle engines to remove carbon deposits. The Brits called it decoking. There was also the “Italian tune-up,” running the engine near redline to burn off carbon deposits.

Modern engines are even more sensitive to carbon deposits on such components as intake valves and fuel injectors. These deposits cause a rough idle, hesitant acceleration, and knocking or pinging, among other symptoms. In 1996, the U.S. Environmental Protection Agency decreed that gasoline refiners must add detergent additives to every grade of gasoline—regular, midgrade, and premium—to prevent carbon buildup.

But in 2004, eight automakers—Audi, BMW, Fiat Chrysler Automobiles, General Motors, Honda, Mercedes-Benz, Toyota, and Volkswagen—deemed the federally required amount of detergent additive insufficient. They formed an alliance called Top Tier to test and certify that gasoline brands have additional detergent additives to keep their engines performing as they were originally designed

In 2016, AAA hired an independent engine testing lab to see if Top Tier gasoline cleans engines any better than non–Top Tier brands. The result? Top Tier gasoline averaged a whopping 19 times fewer deposits than non–Top Tier gasoline after only 4,000 miles of simulated driving. 

For motorists who’ve been using lower-quality gasoline, the AAA study had good news. There’s no need to tear down the engine and decoke. As stated in the final report, “Engine carbon deposits formed when using [non–Top Tier gasoline] can be largely removed by switching to a gasoline that meets Top Tier standards”—though it may take a few thousand miles of driving. 

More good news: The average price difference between Top Tier and non–Top Tier brands was just 3 cents a gallon. AAA also found non–Top Tier gasoline reduced fuel economy by 2 to 4 percent, meaning that the higher cost of Top Tier brands is mitigated by improved fuel economy.

Seems to me like pretty good reasons to stick with Top Tier gas. To find brands meeting Top Tier standards, go to toptiergas.com/licensed-brands.

A note of caution: Don’t confuse gasoline quality (which refers to detergent additives) with grade (which refers to octane rating). Buying a lower octane rating than specified in your car’s owner’s manual also harms performance and could cause engine damage. Buying a higher grade than required is a waste of money.

Veteran automotive journalist Peter Bohr has been writing about cars for more than four decades.

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