The theoretical ideal air/fuel ratio is 14.7:1, by weight not volume. Since a carburetor meters volume by flowing liquid gasoline through a main jet, as the altitude goes up, air density decreases, but the volume of fuel remains the same. At about 3,000 feet altitude or so, carbureted cars experience a noticeable drop in power due to the mixture richening enough to get way out of the stoichiometric band. The higher you go, the worse the performance gets. My 59 Burb with the 1970 307 engine would barely pull Sonora Pass in California at 10,600 feet, and that was with a 4.56:1 rear axle ratio. It ran great down in the San Joaquin valley, but a fishing trip to Leavitt Lake above the pass was a real adventure. Yes, altitude makes a measureable difference, something that electronic injection is constantly compenasting for with mixture, timing, and air management to the catalytic converter.
Jerry
