Originally Posted by 450Donn
You might consider heading north and picking up I90 at Ritzville.
I90 has the lowest grades, mildest weather, and least sidewind effect (typically). Also, disruptive storms are more probably along more southerly routes. Go through Hermiston and the tri-cities and catch I90 at Ritzville to avoid the Vantage Hill, just east of Ellensburg. That hill is the worst pull on the entire I90, as far as I'm concerned, and I've done it end-to-end. It's literally uphill both ways, all day, with a sidewind, in the desert. It might not be the steepest, but it's a killer.
You drop out of the Rockies onto the Montana plains, with options for routes as you proceed east. You can sort things out more comfortably from there as things develop.
Someone said the interstate system is limited to a 6% grade. Perhaps in theory. It was also designed to move military convoys at 100mph. In the west the mountains didn't get the memo. Some of those grades make my ears pop and neck snap just looking at the map. . . .