In a study released at the end of June 2016, the AAA auto club found that gasolines with inferior detergents are clogging motorists’ engines.

Since 1996, federal rules have required all gasolines to contain detergent additives that clean engines. However, major gasoline brands do not stop there — their gas contains extra detergents. These are called Top Tier® gasolines. Top Tier is a gasoline licensing program created in 2004 by seven automakers (BMW, Fiat, General Motors, Honda, Mercedes-Benz, Toyota and Volkswagen) who recognized that EPA minimum detergent requirements were not optimal.

The AAA commissioned testing from an International Standards Organization 17025-certified engine testing laboratory to perform an American Society for Testing and Materials (ASTM) standard test on a variety of fuels and provide the results. The study found that after the equivalent of 4,000 miles of driving, gas without the extra detergents deposited 19 times as much carbon residue on engine intake valves than the so-called Top Tier gasolines. AAA’s full report can be found at

AAA says carbon deposits reduce mileage, increase emissions and lower performance, particularly in newer cars. The extra detergents cost about 3 cents more per gallon, and AAA found that switching to a so-called Top Tier gasoline for a few thousand miles can clean up some carbon deposits.

A survey by AAA revealed that three-fourths of consumers buy gas based on price or on the convenience of the service station’s location. Only one consumer in eight chooses a brand because it has extra detergents.

The Center for Quality Assurance in Midland, Michigan, now runs the commercial Top Tier licensing program that charges gas stations a fee to use the Top Tier seal of approval. A updated list of retail gasolines licensed as Top Tier brands is at