Driving home from the Ouray National in 1985, I came back through Monarch Pass (11,000 feet above sea level) in Old Toby, my '54 Aero. Both of us found that breathing was not as "fulfilling" as we were used to, but there were no real problems with it. Next year, I was in Denver on business and bought a 1959 Studebaker Silver Hawk that had spent all of it's 170,000 miles in Denver. I drove home over Monarch Pass again and the car did well, although the carb was probably jetted for a higher altitude. It had a Studebaker 259 V-8 with a two-barrel carb, 3-speed and overdrive. Needless to say, overdrive was not used on either side of Monarch Pass.