Probably because Chevron was the first to qualify as "Top Tier Gasoline" in 2004, now there are about 30 in the US. They don't get better mileage, they have more detergent(s) to keep crap from building in the motor, which eventually will affect emissions, mileage and performance.
Quote:
Tom Magliozzi, co-host of NPR's weekly radio show, Car Talk, said that using top tier detergent gasoline is only critical on high-end vehicles. For other vehicles, he and another source said that periodic use of a concentrated engine cleaner every 100,000 miles will "often" clean out carbon buildup.
Magliozzi's co-host, Ray Magliozzi, said that in order to be sure of preventing buildup of fuel injectors and valves, motorists should use Top Tier gasoline "at least most of the time".
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