Just so you know, it takes about 6 hours to cook a 20 pound prime rib to an internal temperature of 130 degrees fahrenheit at dead center. That is: if you go Low & Slow. Let's get into the benefits.
Yep, that was dinner at dad & mom's last night, along with two of my brothers and their families. Everyone is headed off in separate directions for Newtonmas, so we decided to all get together and do up a prime rib. In case you're wondering: $150.
Now, let me clue you into something I have found recently with respect to cooking roasts. Most commonly, I do tri-tips, and I do them on the grill. I like the meat medium rare, and it's very difficult on the grill, too easy to overcook. That is, by the time the center is warm enough (125ish for a thin tri-tip), it's brown/grey around the edges, often 1/2 to a full inch into the meat — and with a relatively thin tri-tip, that doesn't leave a lot of tasty pink.
So, I've begun doing them in the oven at 250 with amazing results. Tri-tips take under an hour, I rest them for 10 minutes, then put them under the broiler on high for 2-3 minutes per side in order to get that grilled taste. Best of all, the meat is uniform, pink and tender from center to the very edge.
This prime rib, however, was thick. I also have one of those wireless temperature probes. It started off at 2pm at an internal temperature of 45 degrees and took until 7:45 to get to 130 degrees. We let it rest for 15 minutes, gathered the jus, and sliced her up. Tenderest prime rib I've ever had — which is just another advantage of cooking slowly enough to break down the connective tissue. Here was the result.