Costco gasoline does not have a special chemistry that makes it disappear faster than other legal pump fuel. Energy content is set by the blend: mostly hydrocarbons plus, in most U.S. regular grades, about 10% ethanol (Costco is not ethanol-free).
Why MPG can look different
- Ethanol. Ethanol has less energy per gallon than gasoline. E10 at Costco and E10 at a name-brand station should be in the same ballpark.
- Octane mismatch. Putting 87 in an engine that requires 91 can cause knock retard and a small economy drop. Check Costco premium octane against your manual.
- Trips and tires. Short trips, cold weather, roof boxes, and low tire pressure dwarf tiny fuel-brand differences.
How to compare fairly
Fill from nearly empty, reset the trip computer, and drive the same commute for two tanks. One fill-up is noise. If you want a cost comparison rather than MPG folklore, use the fuel cost calculator with the posted price from a U.S. Costco station.