101 is also a good option
While nothing is as fast as I-5, (except in Tule Fog season), you might wish to consider US-101 through the coastal mountains and valleys. For all intents and purposes it is interstate quality for 90% of the route. Speeds are comparable to I-5 and a Fall trip will be great.
North of Eureka, it slows somewhat going thru Redwood National Park, (oh what a shame!) and then if speed is beckoning, you can cut over to I-5 again on US-199 (joining I-5 at Grants Pass). If timing is still OK, US-101 continues north along the Oregon coast -- a little slower than the California section, but so beautiful!