« on: December 17, 2014, 01:29:28 pm »
Timing looks just about perfect to me if you start heading west on the Northern Tier. You might want to check out snow removal pages for Logan Pass in Glacier NP and Washington Pass about mid-April to see if you can start early or if you need to shift a couple weeks later.
Starting May 15-30 on the NT, guestimate six weeks to hit Glacier (at 50 miles per day), the Park Service tries mightily to get it open by July 4. (Depends on snow pack, of course, but it's usually open by July 15.) Two more weeks to get to the Pacific coast, you'd be starting down the coast in peak summer, call it August 1. Great weather, potentially traffic on the Pacific coast. Take a few days off at the beaches, and you'd be into September by the time you approach Mexico.
The first two weeks of the Southern Tier would be the toughest, because, as Pete says, there'd still be lingering summer heat. However, you'd just about miss the monsoon season in Arizona, and the weather should be pleasant, if dry, by the time you get to eastern Arizona and New Mexico. Fall is going to be prime time when you hit the Gulf coast, barring a hurricane.
The tough part is going to be heading north on the east coast and crossing back to Indiana as winter approaches. You might try to push for more miles to get home quicker, but then you run into summer heat in the deserts. You might want either to skip the Atlantic and come north on the Underground Railroad (or a similar route), or to make this into a two-year trip, picking up the Atlantic from Florida starting around March 2016.