The title of this thread is,
"higher octane fuels to get to 54 mpg."
Guys and girls, we are dreaming. Higher octane fuels will almost never allow us to get to the 2025 CAFE standard.
The standard of today is 35.5 mpg. 54 mpg is an increase of 52.1% over today's reality.
The effect of change of octane number is tightly controlled by our knowledge of the Otto Cycle which has been fully understood by the basic Laws of Thermodynamics.
Increases in octane numbers by themselves do not improve fuel efficiency 100 octane fuel in your car which has been optimised to burn 90 octane will make no difference other than if your car has variable timing sensors in which case you will see fuel use improvements of about 8%.
The real value of high octane fuels is that they allow compression ratios to be increased. The effect of increasing compression ratios can be estimated from formulas developed from the thermodynamic characteristics of fuel compression and burning with well known boundaries.
I have not tried to calculate the average compression ratio of vehicles today but it is certainly above 10 to 1. A lot of cars list values above 11 to 1 and high performance motor cycles are in the 12.5 to 1 range.
The articles quoted in the opening messages of this thread speak of 15 to 1 CR being possible or likely. For this discussion I will accept that estimate.
The important piece of science that comes from this is that raising compression ratios from 10 to 1 to 15 to 1 only provided a combustion efficiency improvement of some 13%; certainly a long way from the 52% required. If today's average CR is closer to 11 to 1 then the efficiency improvement to 15 to 1 is only 8.3%.
Reaching the required CAFE in less than 10 years is going to take an enormous change to the technology mix of our vehicles which really should be the subject of a re-titled thread.