Step 1. Preheat your oven to 325 degrees Fahrenheit for a pot roast from the round; 425 degrees for a pot roast from the chuck; and 350 degrees for a pot roast from the rib. The weight and type of roast determines the cooking time; you should cook a roast until its internal temperature is between 135 and 150 degrees F.For instance, a 2- to 3-lb. rump roast cooks for 1 ½ to 2 hours to medium-rare doneness, the Cattlemen's Beef Board and National Cattlemen's Beef Association website advises.tep 4. Remove the pot roast from the oven when it is 5 to 10 degrees below the recommended final internal temperature. Cover the roasting pan loosely with a tent of aluminum foil and let the roast stand for 15 to 20 minutes. The internal temperature of the roast will rise another 5 to 10 degrees.