Super Six fuel mileage
Best average slant mileage I've personally witnessed was my dad's new '61 Savoy with automatic. It consistently made 21-23, and 24-25 on trips.
Best I ever owned was a '63 Signet automatic. It averaged 19-20 and 23-24 on trips. Short runs under perfect conditions might have made 27, but 30 was beyond reach. My son's stripped '69 Swinger got about the same.
As much as I love slants, mileage isn't one of the things I love, and the alcohol laden slop that passes for fuel today isn't helping.