In this week’s Straight Dope column, Cecil Adams addresses the question of how to choose between premium and regular gasoline. This topic is surely of great interest to everyone as the price of regular gas hovers around $2 per gallon. I thought I already knew the answer: use the lowest grade that doesn’t cause your engine to knock. But that rule is out of date, because today’s engines detect knocking and automatically adjust the timing to prevent it — at the expense of power and fuel economy. So how do you know which grade of gasoline to buy? Cecil offers a rule even simpler than mine: read the owner’s manual.