Spokane to Boise

On top of the steep downgrade just north of Lewiston, you'll have a steep downgrade at Whitebird. From there you'll have some other downgrades between there and Weiser before taking I-84 East to Boise. Lewiston and Whitebird are the two steepest and longest. Go online - Youtube and you can search for both.

We just took 95 from McCall to Bonner's Ferry a few weeks ago. Good road the whole way. There was construction in one place south of Lewiston but the hold up was short.

It is quite a grade into Lewiston but doable, just keep the speed down, use your gears and brake sparingly. Same with the one at Whitebird, I clinbed that sucker with an overloaded underpowered truck.The rest of the road is good and quite scenic.

US 195 to US 95 is a good way. There is a steep down grade north of Lewiston, so watch your speed.
US 95 is a good two lane highway much of the way.