I’ve wondered about this many times before and eventually concluded it was simply to give the perception of gas prices being cheaper than they really were, though I never personally thought it made much sense. A whole cent less…Wow, what a perception. *sarcasm*

Ask Yahoo addresses this question, though.
Why is gas priced to nine-tenths of a cent?

There is no definite answer according to the article, though it does mention several theories, including:

• a marketing gimmick that likely begun during the 1970 “gas wars” and has since become a standard
• the precision of nine-tenths gives motorists a “false sense of accuracy” over their purchase
• a marketing thing (”It looks cheaper”)

Try purchasing one gallon of gas at $2.99 9/10 per gallon, pay with $3, then ask for your $0.001 change. You won’t get it! That small amount of change adds up and only pads the pockets of oil companies and related entities.