Climatologist Brian Brettschneider created a year long, 9,125 mile US road trip where each day is at or very near 70°F (21°C). On his climate blog, Brian explains:

Since you cannot know what the temperature will be more than a week in advance (the current limit of decent forecasts), you have to go with Plan B; that is, look in a climate almanac. The go-to climate almanac for United States data is published by the National Center for Environmental Information (NCEI). The daily normal high temperature is computed for over 8,000 stations all across the United States. [source]

 

The road trip spans a calendar year and on his blog, Brian also created a road trip that follows 80°F weather in the US as well as a road trip that ventures into Western Canada and up into Alaska. Be sure to check out the complete blog post for more insight and information!