Like Michael, I felt that your plans were mostly good, but you have a couple of problems.

There's only one way to be in Seattle by noon, and that's to start the last day EARLY EARLY EARLY in the morning, like 2 or 3 am. My husband and I have been forced to do that (because of rude neighbors in a motel), but we didn't find it fun to be up that early on the last day of our trip and trying to make high mileage.

Great Smoky Mountains NP to Zion NP is about a 4 day drive. Three days isn't enough.

Also on that last two days, included in the first 1100 miles is a nice stretch of 2 lane road, going over a mountain pass to catch I-15 north. On either road (US-89/UT-20 or US-89/UT-14) you'll be hard-pressed to average 40 mph, and they're not roads to do in the dark, either (the scenery is too pretty!). You could conceivably overnight in Boise, though.


Donna