Northern California, but I often have a stop or 3 in Utah, Montana or Washington along the way which obviously complicates things. Thanks though!
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Mike W. Since you're still in the planning stage for your July vacation and don't know where you're going or where you'll be stopping, I highly recommend that you buy a good road atlas that shows all the interstates, the 2-lane highways, all the gravel roads, the federal and state forests, mountain peaks and passes including their elevations, campgrounds, recreational areas, and all the places of special interest. The Rand McNally Road Atlas is excellent when planning a trip because it's so detailed and shows all these things right on the road map including distances. There's a US road map and a detailed road map for each state. Although your trip sounds a bit ambitious for someone who's taking a trip "on the fly", it can be done. You just need to have a clear understanding on where you're going to go at the start of each day and always be mindful of the local weather conditions and terrain, especially, when driving through the mountains and/or through the high country desert. And it doesn't matter if you're driving through WA, ID, WY, UT or Montana. Having a detailed road map will help you immensely, not only during your trip planning process but also when you're on your trip. It might even save your life. After all, you don't want to get caught in a July blizzard at 10,000 feet....not fun! So, please, buy a good road map so you can do a rough draft of your trip and take it with you. You won't regret it.
I've had and used Rand McNally atlases for decades, although rarely these days, I'm mostly here behind the keyboard making plans. We also usually have AAA maps, the wife doesn't like to travel anymore with out a GPS, we have been there half a dozen times or so, so we're not unfamiliar with it all. Certain parts are locked in place, IE reservations at the Old Faithful Cabins, and in the days afterwards. But getting there is still somewhat up in the air.
I'm so glad that you've already been to the Park and are also familiar with the Rand McNally Road Atlas and AAA guides. They're wonderful resources. That being the case, I guess I'm a bit perplexed as to why you would ask about an alternate route to Yellowstone and GTNP from Northern CA. Other than the directions that I've already mentioned, I guess it really depends on whether or not you want to see/visit the Tetons. Seems a shame to miss them when you're that close. And, some of the Forum members have already made some good WY destination suggestions that you might want to consider. Keep in mind, that July is at the peak of the summer visitor season. So you're likely going to encounter some traffic delays, inside and outside the parks. So you may want to factor that into your plans and be as flexible as you can be. Hope you have fun vacation and a safe trip!