Non-top tier gas + additive to save money?
This isn't exactly a question of efficiency as much as it is a question of economics, so I put it here. Typically I would fill my car with Chevron (something about the blue everywhere makes me feel good), but I didn't feel like driving 2 miles just to get to the cheaper Chevron station in the next town so I just filled at a Valero. Typically I avoid Valero/ARCO since it's "lower quality" gas, but I read that the only difference is a small amount of detergents, so I poured 2 ounces of Marvel Mystery Oil in the tank since it's composition suggests that it's a good cleaner.
1 quart of MMO costs about as much as 1 gallon of gasoline, but the additives in gas are well under 1% of the composition, so adding just a few ounces should be bringing the "cleaning power" up quite a bit? So I saved maybe 1 dollar on the actual gasoline getting Valero, and then dumped 30 cents of MMO back in.