I don't want to be the one but;
I have run the the compression ratio up on my combo to as much as 11.3, with cranking pressure approaching 200 psi, and ran it for 16 years on 87E10.
One combo I had went 32 mpg in overdrive with gearing that was making 65=1600 rpm.
So
the point is this, if your low-compression carbureted engine gets better fuel economy with better gas, then the tune is off in the first place.
BTW, IMO, there is NOTHING wrong with 10% Alcoholized fuel. I've been running it in my combo since 1999, same fuel system/same fuel pump/ same carburetor/same tank. and
whereas you said your car runs richer with alcoholized fuel, that is Not correct. Your engine needs MORE alcohol to run stoichiometric than gas. So when you add alcohol to gas, making no other changes, the mixture actually goes a hair leaner. However, at 10%, this poses no real problems.