Would you consider that the EPA mandated fuel mix requirements in the 2 states are different and that the car runs differently on the two fuel mixtures. If she only runs her car to half full before refueling then it would take a few fills to get fully to the performance at each location.
If the car was originally purchased in Colorado, it was probaly adjusted for high altitude driving, in which case running it at sea level, would make the car run too rich. Back in carberator days they would advance and retard the timing to make the adjustment to and from altitude.
What is your exact question and what TWO states are you referring to???
I think there’s a good possibility that the special gasoline formulation sold in California to combat smog may cause reduced mileage. Ethanol blends deliver worse fuel economy because ethanol has less energy per gallon than gasoline, among other factors. Gasoline costs much more in California than the surrounding states, partly because of the special formulation. It improves air quality, but perhaps at the expense of fuel economy.
That seems plausible to me if there is a minimum requirement that is allowed to be exceeded and Shell gas exceeds it.
However, the state that I spent 3 years in was Kansas. The last 6 months I lived there, Shell opened a station nearby. When I began fueling with Shell gas my gas mileage returned to 32/24! I have fueled with both Shell and non-Shell gas in Ohio, Iowa, Nebr.,and Kansas always with similar results.
There is probably some truth to that. We live at a higher altitude, and one oil company puts premium in both the regular and pemium tanks at the service station. They get fewer complaints that way. My 1980 Olds 350 could not use the regalar gas of the rival oil company at this altitude (pinging) so I switched to the other who spiked the regular.