 it retards the timing to protect the engine. The problem is that this retarding timing is reducing power. So the money he was saving by buying 87 instead of the 91 that the car was designed and required to use was more than offset by the fact it cost between three and four miles a gallon. Went to 91 and within three tanks, the car was getting its rated gas mileage. But we see that happen with real gas versus, you know, ethanol adulterated E87. Uh,