Hi,

40 is higher in altitude and may be cooler from Flagstaff on east for a ways, but it can still be hot! If you've driven 10 then take 40. (Mark is right about the long hill west of Flagstaff. Take it slow and don't cook your car). There are some interesting things to see along the way like the Painted Desert/Petrified Forest in Arizona and Acoma Pueblo in New Mexico for starters.

If you are in Winslow, AZ around noon, get off the highway and drive south into town. Find the old hotel along the railroad tracks and have lunch, and an interesting history lesson.

Craig Sheumaker
co-author of the travel guide: America's Living History-The Early Years