Place 3 to 4 pound rib roast, fat side up, on a rack in an open roasting pan. Insert meat thermometer into thickest part of meat. Do not cover. Roast at 325 degrees.
For a 4 to 6 pound roast, allow 26 to 32 minutes per pound for rare, 34 to 38 minutes for medium and 40 to 42 minutes for well done.
For a 6 to 8 pound roast, allow 23 to 25 minutes per pound for rare, 27 to 30 minutes for medium and 32 to 35 for well done. For easy carving, allow roast to stand in a warm place for 15 minutes after removal from oven.
If you use a meat thermometer, it will register 140 degrees for rare, 160 degrees for medium and 170 degrees for well done. Since roasts usually continue to cook after removal from oven, remove roast when thermometer registers about 5 degrees below the temperature of doneness desired.