Hm. I guess it’s still possible that the difference would be noticeable. I would use the “cheap stuff”, it’s a KIA, after all. Gasoline suppliers change their blends seasonally, this might be a factor. Reformulated “clean air” gasolines may be as well.
Good point. Lots of folks I know claim lower gas mileage on the oxygenated “winter gas” you see in most of the US, so perhaps his Kia is noticing the difference there.
Personally, I take a 10% gas mileage hit in Ohio winters, although I suspect some of that may just be increased warm-up time for my car… your engine is most efficent when it’s near 200 degrees, not 12. Also, my transmission tends to lock out overdrive and refuse to lock the torque converter until it hits a certain temp. It’ll gear up and lock down 2 miles into my drive in the summer, but it seems like it can take 4 miles on a sub-freezing day.