Ebo, as already stated the percentage of ground is what determines the fuel gauge reading. Our cars were made to handle up to 10% ethanol. Many gas station pumps are now 12 to 15%, even if they say 10%. The feds made this change allowable several years ago. The GM owners manuals used to say 10% and the Feds required a recall to "correct" the issue. GM simply did a recall to the owners manual. They replaced the page with one that said 12%.
Different fuel compositions change the fuel conductivity. There are acutally electronic tools that use resistance to determine ethanol content. The ethanol is making the fuel sending unit more conductive, increasing the ground path, causing a reading lower than it would be otherwise. Without the ethanol, the resistance is normal, and the gauge reads properly ( or close to it ).
Keep in mind other factors tie into this. Fuel is custom blended on average every 3 weeks to adapt for changing climate. This includes temperature, altitude, humidity, and other factors. So different areas of the country may cause this problem, and others not. Different times of the year can also have an effect. AND different units can be affected differently with the same blends. Quite a odd blend of chemical and mechanical soup.
The EPA uses Indolene instead of gasoline to do all emission testing on new cars in controlled conditions to determine emissions and mileage. This keeps a fair compairison of all vehicles without variations due to different fuels.
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