The beef rump roast is a popular meat choice, due primarily to its excellent taste and ability to feed large numbers of people. Fortunately, if you cook a 1-2 kg (3- to 4-lb.) rump roast for the proper amount of time, the result will be a mouthwatering entrée.
Cook by the 500g
Rump roasts come in a bevy of sizes, so knowing how long to cook each 500g serving and to what extent is important
If your roast is given a "prime" or "choice" rating, you can plan to cook it for 25 to 30 minutes per 500g (pound) for rare, 28 to 30 minutes per 500g (pound) for medium rare, and 31 to 36 minutes per 500g (pound) for medium meat, all with the oven set at 160 degrees C (320 ºF). Give it closer to 40 minutes per 500g (pound) for a well done entrée.
To ensure your rump roast is done, regardless of how long you cooked it or at what temperature, check the internal temperature with an instant-read thermometer. You should cook your rump roast until the internal temperature of the rump roast (while it is still in the oven) is about 50 degrees C (120 ºF) for rare, 60 degrees C (140 ºF) for medium rare, 65 degrees C (150 ºF) for medium well, and up to 75 degrees C (165 ºF) for well done results.
Once the desired temperature is reached, turn off the oven and let the rump roast sit for 20 to 30 minutes, which will in turn help it cook for a bit longer, thereby reaching the optimal temperature.