High Altitude

Yes there is, it's the downhill side of the mountain you went up...

Or do not tow it should help the gas milage

A friend of mine told me that you can expect lower fuel economy and you'll see more smoke out the tail pipe when you travel in higher elevations. Personally, I haven't experience this yet. However, next summer I plan to travel into the mountains and that is why he told me what I can expect in terms of fuel economy and the smoke issue.

Well yeah, you haven't experienced it yet because they way you've worded it, you haven't been to the mountains yet.