I've been using Costco gas for years in Texas and now Chicago. My mileage hasn't decreased.
Robert if you don't he does limit it'll change. The faster you drive the faster you burn gas.
In the winter all stations in cold areas have an additive added to prevent freezing. This causes the gas to burn slightly faster but should result in less than a 1 mpg difference.
No I have not noticed. Drive same speed and did travel 4 hours straight and gas seemed to burn same.
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