I gave up on my book because after a couple decades I didn't have a good answer. Sure I could brag about the one full-up where I got 57mpg, and I knew about the one where it was only 36 - who cares.

Back in the 1930s the book was probably useful because changes in mpg were a sign you needed to do a tuneup early. But back then everybody knew a tuneup was every few months and once in a while something went off in between. Cars are a lot more reliable.