The Onion

Altitude has a significant affect on engine performance [1], regardless of your opinion on transmissions. Conventional wisdom dictates declining carry capacity per altitude gain. "Note: For high altitude operation, reduce the gross combined weight by 2% per 1000 ft. (305 m) starting at the 1000 ft. (305 m) elevation point." [2] As does incline, which if you read my comment carefully you will notice I mentioned.

I'm not sure you're an authority on what folks in the American Midwest are or are not towing with cars, but I will note that automobiles in North America have one rating, nationally. There's no regional tow rating for Rockies vs Flats, or cold weather performance in Montreal vs Florida.

As with most all things in life, the answer lies in a complex host of variables, not just one singular difference. Just trying to be informative, there's no need to be defensive.

[1] https://www.aamcocolorado.com/high-altitude-car-maintenance/ [2] https://www.cars.com/articles/should-your-pickup-tow-less-at-altitude-454166/