I'm not a car knowledgeable person and could possibly be over thinking this. I put premium in my car. I've gotten my car to a consistent 26 to 28 mpg. I don't know when my mpg went down but I looked yesterday on my car and it's down to 18mpg. I haven't changed my driving habits. Any other ideas then me jumping to the conclusion the quality of gas has decreased?
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The difference in premium and regular has to do with octane rating. Higher octane means the gasoline is suitable to engines that have higher compression ratios. Higher compression ratios are needed for higher fuel efficiency. It has nothing to do with the heating value of the gasoline, but with its resistance to knocking (compression ignition).
To get a huge drop in mileage such as you've mentioned, I would check the engine to see if something has gone out of whack with its functioning. Or possibly, the supplier has gone to a heavy ethanol-based mixture. Ethanol is chemically like gasoline wedded to water; it has very low heating value. Too much ethanol can be deleterious to an engine's fuel supply system. Either way, you really should take it in for inspection. Fuel economy is a great metric to diagnose the health of your engine.