It's all about the RVP, reid vapor pressure. In 1992 the EPA set the max allowable RVP of 9 psi to reduce evaporative emissions, but gave E9-10 1 extra psi. E15 didn't exist so it was not included.
E15 RVP is barely higher than E10. EPA is just going to give it the same 1 psi allowance as E10 to E15 so it can be sold to all cars year round.
Some state don't allow the 1 psi waiver and the refiners have to remove more higher volatile products in the gasoline to say under the 9 psi.
E15 has the highest RVP of any blend, it drops after that matching gasoline at E50. E85 RVP is 5 which cause the cold start issues. E70 is 7.
https://ethanolrfa.org/wp-content/up...6_12_Final.pdf
Ethanol Subsidy - aka Blenders Tax Credit. Just trying to figure out how this thing works. For every gallon of fuel a blender mixes they pay $0.184. For every gallon of ethanol they use the get $0.45 tax credit. So a gallon of E10 blend, net tax is 18.4 - 4.5. 13.9. E30 net tax would be 4.9, E50 tax credit of 4.1. E85 tax credit of 20 cents.
I didn't know there was an extra 18.4 cents per gallon of tax before the 29 or 31 I pay at the pump.
What I don't understand is why some ethanol plant in Iowa sell E70 during the summer. Unless the natural gasoline (70 octane) they are buying to mix it with is cheaper than their own ethanol.