best route to portland oregon from lake tahoe ca.

If you are looking for the nastiest, boringest route it is I-80 to I-5 straight into Portland. Shorter and not too hard is 395 to Susanville to the 49 to the 89 and up the i-5 from Shasta. That is likely the route your GPS is using and is a good road with no big climbs. a Much nicer drive.

But probably to be considered is if you are going to do the whole drive in one long painful slog. If you are planning to stay somewhere on the way, I think the recommendations would change.