I fly at an elevation of 7000 feet. I have noticed that I seem to get about the same prop speeds on my engine/prop combinations as everyone else flying at lower elevations. I assume that two factors are working at the same time:

1. The air is thinner, so the engine has less power.
2. The air is thinner, so the prop has less resistance.

These two factors seem to equally offset eachother, so props seem to turn the same speed. independent of elevation. Needle valves do need to be set a little leaner here (I think).

Any comments?

the air is thinner, theres less air to push
try flying on really cold days

I flew on a 30 degree day from previously flying in 60 degree weather and the difference was unbelievable! It was like night and day. However the same principles apply to higher altitudes. If you fly at 70 degrees at 2000 feet and then go to 70 degrees at 7000 feet, it will be the same as going from cold weather to hot weather.

At higher air density, wings have better lift and props generate more thrust without necessarily increasing RPMs. Anyone notice whether their engines turned higher at cold temperature than at warm temperatures?