It’s important to remember that a carburetor is a Fuel / Air metering device. Most likely it was too rich with the .101 jets, but ran a bit better because it helps mask a bad timing adjustment.
The timing needs to be spot on before any changing of carburetor jets happens. It needs to be set properly by someone who understands what is supposed to happen. Centrifugal advance, vacuum advance, and initial are all as important as total advance. A good advance timing light is all you need, along with some way to know engine RPM.
Any re - jetting of the carburetor needs to be done with the help of a fuel / air meter, preferably on a chassis dyno. Otherwise you are just asking for trouble.
15 MPG is about right around town. With a Holley 600 CFM 4 barrel carb I averaged just under 20 MPG on a trip from Dallas to San Francisco and back with the AC on most of the time. Not sure if an Edelbrock carb is capable of that kind of performance but it ought to be close.