"What is the criterion for the best altitude for a cross country flight?" - Archer H.
“As an airline pilot, my major concern was to provide a smooth ride for the passengers even at the expense of efficiency. If you think about it, that should be a large factor in our general aviation planning as well.
We don’t want to shake and bake our passengers by being down low in the bumps when we could have climbed a few thousand feet and kept them cool and smooth but had a little more head wind. Most of the time, the amount of wind change from say 3 thousand to 6 thousand is not so great that it will change our flight time by more than a few minutes. But if you can get a smooth ride by making that change you and your passengers will enjoy the flight much more.
Another reason to think higher rather than lower is that most of our non turbocharged airplanes get their best true airspeed between seven and eight thousand feet. So everything else being equal, this would be the place to be on all cross country flights.
Then there is the terrain issue. If I am flying over uninhabited and/or rough terrain, I want to be high so I have a better chance of reaching an airport or at least civilization if I have a problem. I am very happy to be a little slower but well above the rocks.
Sometimes our choices will be limited by the weather. If we are VFR only pilots the cloud bases will be a limitation. But, If the clouds are scattered to broken along your route and you can safely climb on top, you will almost always get a better ride. Just be sure to comply with all cloud clearance rules and don’t get caught on top of an overcast. If you are an IFR pilot the freezing level may limit your altitude choices. And of course, we all have to obey the oxygen required rules if we plan to go above 12,500 feet.
So unless there is a big wind penalty, my first choice is a cruise altitude of 7 or 8 thousand feet to obtain the best true airspeed. If I need to climb or descend to keep the ride smooth I will unless I am over hostile territory, then I won’t descend.
In my view, higher rather than lower cruise altitudes give a pilot more options.”