Mayfield, Ky to Portland Oregon

My personal preference would be to take the I-90 route. Mostly because I've found that the road conditions and truck traffic were better on I-90 than I-80. To be fair, it's been a few years since I've been on I-80 west of Lincoln. There's been a lot of road construction on I-80 in Omaha the last few years and it's still going on, but it's not too hard to navigate; get in the middle lane and stay there all the way through Omaha. The previous poster is correct: early to mid August is a bad time to go through South Dakota. Not only will the motorcycle traffic be really heavy throughout the state, but finding a camping spot in the western part of the state will be difficult to find and, if you do find one, it will be a minimum of 2-3 times the normal value; I'd be surprised if you could find a spot for less than $75/night. Once you reach the Missouri River at Chamberlain (the view at the rest area is great)the terrain changes significantly and becomes a lot more interesting. There are also a lot of tourist attractions along the way, some good, some not so much. Hope this helps.