Went to Costco's site, clicked on Membership, then Executive membership, then further down in the small print "membership benefits and Costco services."
"In the U.S., Rewards are not calculated: (i) on purchases of cigarettes or tobacco-related products, gasoline, Costco Cash Cards, postage stamps, alcoholic beverages in certain states"
I had the same thing happen to me as all the other posters. The cashiers scan my card and tell me I'm going to get this huge check, totally worth it, etc. I go up to customer service to ask about it and they explained gas is not covered although it's all together in their computers. We had to figure ourselves how much we spent on gas to remove that total. The cashiers won't tell you that.