For what it's worth, RockyMtnMerlin, a naturally aspirated car won't start feeling the altitude till around 4,000'. Back in the days when I was backpacking, I believe I felt the altitude starting somewhere around 7,000', but it may have just been hills in the San Juan Mountains. My home elevation (Farmington, NM) is 5,000', so maybe I was less sensitive than some.
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