Thank you. So that says if we have two engines working properly and one is desigend to run at 30% lower compression, it will be 9% less fuel efficient.

Then I need to take account of the losses in my engine, be it valves rings or whatever.

In very simple terms , I remember from school that

P*V/T is constant or P1*V1/T1=P2*V2/T2

where P1 V1 and T1 are uncompressed pressure, volume, temperature and P2 V2 T2 are compressed.

So it seems to me that if my pressure P2 is 30% down, then my volume V2 must be 30% up, asssuming temperatures aren't much different. BUT of course the compressed volume V2 is fixed by the metal around it, EXCEPT that some air fuel mixture is leaking out. And so it must be that 30% is leaking out since volume is 30% up. Which means 30% of my fuel is wasted.

On top of that, I lose 9% because of what you say about the inherent lower efficincy of lower compression.

So I think it is ballpark 40% lower MPG.

Does my simple stuff make any sense?