See, I’m a Costco member, but I don’t bother with the gasoline. Why? Because at the Costcos in my area, the gas lines are always so damned long. Yes, their gas is usually signficantly cheaper than the local average, but I find that if use GasBuddy.com or other similar, I can find other local stations with prices comparable to Costco’s or perhaps only $0.05 more per gallon... and no lines. If I’m buying say 12 gallons, it makes sense to go to that other station. Saving $0.60 total by waiting around at Costco just isn’t worth it.
I used to use GasBuddy but found Costco to be significantly cheaper. Their high test sometimes is $ .60 cheaper and their regular about .50 cheaper. The difference probably varies by area and maybe proximity to the refinery.
We have two Costco locations nearby. One is by an interstate and the other about ten miles away. The one close to the interstate has very long lines. The one located away has short lines, if any. Your Costco may be near an interstate?