It depends on the location, season and duration of the tour. The most versatile bag we have found is a light synthetic bag rated at 20F and weighing 2 pounds. It is not made anymore, but there are similar models on the market. It is the one we have used on all our long tours, including a ride across the U. S.
You will hear a lot of folks who use down bags due to their light weight and compressibility. The new synthetic bags rival down in warmth, weight and compressibility. Synthetic bags will keep you warm when they are wet, dry quickly, and are usually less expensive. I had my synthetic bag hanging over my bike to air out, and a dog came up and peed on it. The campground I was staying in had laundry facilities, so in a couple of hours I had a clean dry bag.
Best wishes on your ride!
This will give you a sense of scale. The blue compression sack with the tent is the same size as the yellow/green sack with the sleeping bag.
Use compression sacks for your tent and bags. The blue bag is the tent and rainfly, green bag is sleping bag, Orange is Thermarest pad, and small gray bag is sil-nylon ground cloth, other gray bag is compressible pillow. All fit easily in Ortlieb Rack Pack.
The Rack Pack is waterproof and keeps gear dry. It is also a nice compact bundle. I prefer not to have a bunch of loose item strapped all on the racks.
