I vaguely recall reading about the origins of this practice somewhere. As far as why it is still practiced is purely for competitive reasons. The dealer may profit more from each gallon sold if he/she rounded up to the next cent (say $1.60 vs. $1.599), but he would be competing against the dealer across the street who is charging $1.599, and would therefore sell fewer gallons.
I believe it’s a marketing ploy. The large numbers might say $1.57, but it’s really 1.58 (if you don't believe me, buy one gallon and *try* to get .001 change!). Anyway, people are probably more likely to buy gas from a station where they can save 15 or 20 cents on a tankful.
What pisses me off is that it really isn’t $1.599. It’s $1.60. They don’t count the .9 cent. When gas was $0.999, I got 10 gallons of gas. Should cost me exactly $9.99. Nope. 10.000 gallons of gas cost exactly $10.00. What a ripoff!
And, having known a gas station owner personally, let me point out that they really don’t have the option of bumping up the price at their station. That’s set by the franchise, and in the gas business, the franchise is GOD.
For the record, I did attempt a message board search on this subject specifically, but came up dry. So maybe I’m not merely lazy, just not too thorough. And I do remember (many) posts about the 9¢ deal; I had figured the gasoline practice I brought up would be different. I never figured psychology was a relevent factor at all, since everyone I know just automatically rounds up the price anyway.
All I know is that gas around where I live is $1.839 and that really pisses me off. A year ago gas was $0.899 for a couple weeks. What the hell is goin’ on here?