I'm making tri-tip in the oven and the recipe is for 2-3 lbs, but I'm making 12-15 lbs. Do I have to double the cooking time? 2-3lbs cooks in 40 minutes for medium.
In an oven, bake the tri tips for the same length of time, whether cooking 3 pounds or 13. Leave space between the beef roasts so heat can easily reach all sides.
Use a meat thermometer to determine when the tri tips are at the correct temperature to remove from the oven. Since the internal temperature of a roast continues to rise AFTER it's removed from the oven (or the smoker or the grill), it has to be taken out BEFORE it reaches done temperature.
Rare 130ºF Remove at 120-125ºF
Medium Rare 140ºF Remove at 130-135ºF
Medium 150ºF Remove at 140-145ºF
If the tri tip is a little underdone, it can always be recooked to the desired point of doneness. But if it's overcooked, well, you can't fix that!