.boB

I do. I live in central Colorado, at 6,200' ASL. I commonly drive up a lot higher than that. A normally aspirated engine will lose about 2-3% of it's power per 1,000 Feet of elevation increase. Fewer oxygen molecules per cube, less power. But a well designed turbo system will gain that back by maintaining fuel charge density.

Last month I drove out to SC and back; I-70 east, and I-40 west. Empty, no load. Pretty much 75mph all the way. So it was an easy cruise that didn't strain the system at all. Fuel mileage didn't change much with the altitude. Because of the turbo, manifold pressure - and efficiency - doesn't change much with altitude. Manifold pressure was about the same all the way across the US. Up above about 10,000' I can feel the difference. Otherwise it runs pretty much the same regardless of altitude.

Pulling a load is a different story. At high altitudes you can run out of capacity, and start to lose power. I only tow a light trailer. Going from Cheyenne, WY, to Laramie, WY is up over a big hump, well over 10,000'. Because it's a light load, the truck doesn't strain, and easily maintains a steady 75mph. But I do see a little more smoke on the way up than I do when empty. I have not compared fuel mileage on the way up the hill.

I used to drive an '08 Mustang. Interestingly enough, that car got much better mileage above 4,500' ASL; that seemed to be the magic number. But it was more fun to drive at lower altitudes.

Chopper1967

My 03 quad cab dually has Stock 235/80/R17. My tires currently are Michelin LTX M/S I'd like to go to a more aggressive tread without too much noise. I would like a taller tire but not crazy tall.
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deere country

The pass east of Laramie is not 10'000 feet at all. It's more like 8600-8800 feet, how would you explain all the trees their if it was well over the tree line.

But yes the simple math on a natuarlly aspirated engine of any fuel is 3% loss per 1,000 feet of elevation.