There is a possibility I'll find myself in the southwest USA later this year (their autumn), and I'm contemplation a short three or four day tour out of Las Vegas. I've researched the rentals and stuff, all I need is the finances. The one thing that's nagging at me is the relatively huge distances I'll be taking on. I know from experience (a long time ago) that interstate travel means a lot of straight slab between destinations. What I need to know is whether it's feasible to average the fairly high state speed limits in normal traffic conditions. Is it practical to maintain 75mph+ averages on rural interstates? For hours on end? I'll admit that I'm no ironbutt rider. 500km a day is what I'd normally manage on tour but rarely on freeways. The other thing is altitude. Some roads exceed 3000m above sea level. I don't suffer from altitude sickness but I've never ridden that high. Anyone had experience of these conditions, and got any tips to offer?