This might be a bit of a long post, but I wanted to share a few things from my experiences along the Western Express route.
The Western Express is my favorite ACA route, having travelled it twice and toured quite extensively in Northern California, Nevada, Utah, and CO. Like John said, leave SF mid-May. You will experience temperature variations right from the start. SF will be quite cool--usually in the 60s F, but the California central valley can have mid 90s F in mid to late May. The good news here is that the humidity is very low, so it isn't nearly as bad as 85F in the mid-west and eastern part of the country.
The elevation at Sacramento is 50 feet. You will begin climbing the Sierra Mts. just east of there, and you will be mostly climbing until you reach Carson Pass (8574'). This is the easiest pass over the Sierra Mts. Needless to say, the temperature will be quite cool in the mountains, and you can possibly experience some light snow squalls. It's a beautiful journey through the Sierra Mts. with a 20+ mile decent into Nevada.
The ride through Nevada in late May and early June is beautiful! With snow on the many mountains, wild flowers everywhere, and the snow melt creating temporary lakes and streams--it's the best time to enjoy Nevada and Utah. However, these two states heat up quickly in June and the distances between available water sources can be extreme. Hydrate often and carry plenty of water.
There are 13 mountain passes across Nevada, with great, flat basins in between. (Some days you might climb four of these passes). The joke is, that there are only three trees east of Fallon. Not quite true, but there aren't any trees across the vast basins, with a few on some of the passes. Once you leave Fallon there are three towns and two former Pony Express stops (turned into restaurant/bar) the entire way east across Hwy. 50--the "Loneliest Road in America". It is 60 to 85 miles between these services. Be smart and use common sense and you will enjoy this region tremendously.
East of Middlegate (free camping, "motel", restaurant/bar--and very friendly folks) there is the "Carroll Summit option". It is a lovely 12-mile climb over the summit, but it will afford you some of the most beautiful scenery on your trip across NV. You can also choose to camp at the top. Just remember, when you leave Middlegate there are NO services/water and no houses until you get to Austin. I've ridden both options, and Carroll Summit is the prettiest.
If you escape Nevada with temperatures only in the 80s, then count yourself lucky. It WILL be hot in Utah. You WILL experience 100+ degree days. Ride in the morning hours, hydrate often, carry plenty of water, and don't be a hero. Take your time through Utah. It is my favorite U.S. state for cycling. If you have the time and inclination, I would highly recommend that you get off the ACA map a little and visit Zion National Park (excellent!) and even the north rim of the Grand Canyon (a 2-day ride from Zion NP). Take time to explore each of these exceptional parks. You can easily get back on route after experiencing the Grand Canyon.
The scenery in Utah is other-worldly, with magnificent red and pink rock formations, pine forests, excellent roads, and beautiful mountains. Some of the climbs can be challenging, with 14% grades that can go on for several miles. (I met a physical education teacher from New Jersey at the top of one such brutal climb. He was gazing over the edge of the summit and decided that he was going to end his cross-country journey at the next town. Myself and a couple from the Netherlands offered him a pep talk and convinced him to continue on. He was fine after that and never regretted his decision to continue. We rode together for the next 9 days into the middle of Colorado, when we headed in different directions.)
Boulder Mountain is a steady 13 mile climb to a beautiful 9600 ft. pass with a breath-taking view to the east. There are many places to camp on Boulder Mt. On the other side of the mountain is beautiful Capital Reef National Park (good camping).
Another option from Hanksville, UT is to turn north toward Green River (camping/motels/food) and then head east toward Moab and enjoy Arches National Park and Canyonlands NP. These two parks are also awesome and are not along the standard ACA route. Moab is a great town, but lots of summer traffic. From Moab you can continue south to Monticello and get back on the ACA route into Colorado.
The mountain passes in Colorado aren't nearly as difficult as the Sierra Mts, some of the Nevada mountains, or Utah. Usually just a steady 6% grade. Expect temperature extremes in Colorado, due to elevation. You can ship your cold weather clothes home or to a friend when you arrive in Pueblo, CO. Hot and humid going east, but a great adventure across small-town rural America.
I hope this gives you a few ideas.
Have a great and safe journey,