Kroger Co. and Costco Wholesale Corp. are consumers’ favorite places to shop for gas, when indexed by number of locations, according to a Market Force survey released last week. About one-third of the survey’s 4,500 participants from across the U.S. and Canada said they most often fill up at a supermarket or mass merchant, while 56% still prefer traditional gas stations. While Shell was the most frequently cited fill-up spot, Costco rose to the top, followed by Kroger, when indexed for the number of locations. Convenience and price were the most popular attributes consumers cited as important for a fuel retailer to have, and cleanliness also ranked high. Costco was voted as having the most competitive pricing, and as having the cleanest facilities of all the fuel retailers. Kroger scored highest in the rankings for having good promotions/discounts and for having convenient locations. Wal-Mart and Sam’s Club had the lowest scores in several attributes.