Whether or not the decision is political, necessary or whatever..
An increase (or loss, however you want to put it) of 5 or even more miles per gallon due to 5% more ethanol content is absolute nonsense.
Yes, consumption increases between E5 and E10. By 1%-3%. Just think straight for a second, there’s cars that run on E85. They have a 15% to 27% higher consumption than a pure petrol (no ethanol) car.
Here we are, talking about a 15% higher consumption through a 5% ethanol increase.
What does increase consumption by that margin is weather. The engine is colder, stays longer cold, so it runs richer for longer. Heating and heated seats come on, more fuel burned. Battery cold, so alternator increasing resistance for more and longer.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.