There are a lot of sites on the Internet that claim to be able to tell you which gas stations in your zip code have the cheapest gas. Some take the word of station owners or rely on volunteers to call in each day, or every time there is a change in price.
  Station owners might not call with every price increase; volunteers can get sick, be out of town, get lazy or forgetful.
  But there are several sites, among them MSN Autos, http://autos.msn.com/everyday/gasstations.aspx?zip=&src=Netx , that instead get their information from OPIS, the authoritative Oil Price Information Service.
  OPIS gets its information from credit card data—how much real people are paying per gallon at more than 120,000 gas stations nationally—each day.
  Scott Ehlers, manager of the MS Autos site, says that because his site is part of a popular Web portal, more people access his site’s gas price data in a day than visit any other gas price site “in a month.” He won’t share the actual number of site visits.
  OK, the Redmond, Wash.-based Microsoft site is a biggie, but ....
  At any rate, the site is very easy to navigate; the maps provided with each fraction-of-a-second search are clear, with nice, large type, and the list below the map spells out exactly where each station is and what it charges for gas.
  If they’re timely and accurate—and I’d like to hear your evaluation of MSN and other gas price sites—I guess they could save us a fair chunk of money.
  And if some stations are consistently lower than those around them, that would be good information to share, too.