650 Red Rock
But, the cross-country trek is an admirable undertaking, and one of those trips where the journey has rewards that can't be measured by scenes-per-mile.
Anyway, what you'll want in a sleeping bag is something very light. I assume you're pedaling from the East Coast to the West, so by the time you hit the highest terrain—in the Rockies and points west—it'll be July. So I expect you might run into temps around freezing (at night), if that.
So a bag rated to 32 degrees should do nicely. My first choice would be something in down, as that material compresses better and is simply more comfortable than synthetic. You could, for example, pick up a Western Mountaineering HighLite ($220, www.westernmountaineering.com). It weighs only one pound and packs down to about the size of softball. True, the HighLite is rated to 35 degrees, but I still believe that would be fine for your trip. Another good super-light choice is Feathered Friends' Grouse ($295, www.featheredfriends.com), a women-specific 30-degree bag that tips the scales at a sprightly one pound five ounces. A more affordable down choice might be Moonstone's 650 Red Rock, which uses good-quality down but stuff that isn't quite as dear as the filling in either the HighLite or Grouse. The Red Rock is rated to freezing, weighs just a touch over two pounds, and sells for $170 (www.moonstone.com).
On the synthetic side, REI makes an excellent 35-degree bag called the Nooksack UL, with uses down-like PrimaLoft for its fill to create a very soft, compressible bag. It goes for $159 (www.rei.com). Or, Mountain Hardwear's Polarguard Delta-filled 1st Dimension is rated to 30 degrees, and sells for a reasonable $155 (www.mountainhardwear.com).
So there you go. Hope you have a great trip! Be sure to cut through Glacier National Park if your route allows—it's well worth it. And pack dog repellent for Kentucky...
Read more sleeping-bag reviews in Outside's 2004 Buyer's Guide .