I will probably get some haters and non believers with this claim but I once changed a rear end for a long highway drive to a 2.56 from an automatic. Was running an M20 Muncie behind a .030 over 400 with a Holly 750. The cam had an advertised lift of 467 and duration of 313. The setup was completely incorrect for any normal stop ad go driving but was happy on the highway. Don’t be hating but, from PHX to Sacramento I figured that it achieved 22mpg. Mostly flat road, gears, gears, gears!