Dear Craig,
Outstanding mileage indeed. Saturn normal real world mileage (gasoline engine, petrol) is about 38 mpg.
I did an engine swap in my Mercedes 89, turning mileage from 26 to 35 mpg, using a Nissan non turbo engine. Car is heavier, though, but is still running on the hands of my son. The smoothest car ever build by Mercedes.
Here is my page of the swap:
https://sites.google.com/site/rd28engineswap/home
It would be very nice if you can publish something similar.
Best,
OldBeaver