I was doing a 40 minute - 1 hour commute both ways to work a couple of years ago and thought - Costco gas might save me some money (15 mile detour).

What I found was that I was eating up gas just to get there and the venture was costing me money. In almost all cases you should go with the cheapest gas directly along your route for the savings to make sense.

Costco is great for food as well with some deals so fill up while there of course. But I'd say never go out of your way to fill up there (like on a road trip) unless you know the math of your specific car and what detour limit makes sense.