Portland OR to Heyburn State Park ID

No reply yet so I'll give my $.02 worth.

I-84, I-82 and pick up US 395 in Pasco, WA. Few mile pull after you cross into WA, maybe 5%, nothing to alarm you.

Taking WA 26 east into Colfax, WA is a decent road, no excitement here.

From Colfax on WA 274 I can't help you.

Have a good trip and hope someone else chimes in.

You aren't crossing any mountains. Grades result from crossing low rivers, such as the Columbia and Snake. And lesser grades in the rolling hills of the Palouse - ancient wind blown sediments.

I84, 82, 90 and US395 are freeway. The rest 2 lane. US12 is the main EW road in southern Washington. US95 (and 195 in WA) is the main NS route. 26 is the main access to Colfax, Pullman (Washington State U) and Moscow from Seattle.

Going all the way to Spokane maximizes the freeway driving.

The easiest way, despite being more miles, would be to take I84 to 395 to I90, then south on US95 in Coeur d'Alene. Despite being a 'highway', SR27 in Spokane is more like a city street until you get out in the country, then it is a 2-lane windy road. US95 south of Coeur d'Alene is basically 4-lane 65-70 mph almost freeway. You won't be on SH5 east of Plummer for long, but it is a 2-lane road, and there is a downhill grade (Pedee Hill) just before the turnoff to Hawley's Landing. It's not that steep or long, but you do have to slow down at the bottom to turn left.

It's gorgeous country. Consider stopping in Coeur d'Alene to do the tourist stops. A little north is Silverwood Theme Park, if you're interested.

We took the "long way" I-90 to 95 and it worked out great. And not much longer at all. Great advice. Thanks!