Originally Posted by cyccommute
I would say that, for most of the country, a very light bag will do nicely...especially for a June start. The place where you'll likely have problems will be in Colorado. West of Pueblo you start into real mountains where the summer temperatures can dip below freezing, depending on cold fronts. Wyoming and Montana are going to have similar temperatures, although not as extreme.
My suggestion would be to carry the 55 degree bag and look for a liner when you get to Pueblo. You could rent a car in Pueblo and drive up to Colorado Springs (about 45 mile north to the REI), if you can't find anything in Pueblo. That way you don't have to huck something of limited use for 2000 miles.
So that the whole trip wouldn't be wasted, you could also go visit Pike's Peak (hint: it's that mountain you see to your right and you've been seeing it since about Eads
). Worth the side trip. If you don't want to drive to the top, there's a cog railway in Manitou Springs that's worth the money too.
I think this is exactly right, and you won't have any problem finding a fleece liner in Pueblo. Or buy one now and have someone mail it to you there. With a June start you will be fighting some hot nights, try a coolmax or silk liner as your sleeping bag for those nights, it's actually cooler that sleeping without anything, wicks away moisture.