I am cooking an 18 lb. prime rib Christmas day and then traveling an hour and a half to serve the meal. At what temperature and for how long should I reheat the prime rib before serving. I don't want to overcook it.
It's really difficult to give good numbers for this, as it's a factor of the shape of the item, what temperature it was when you started, etc.
If you're okay with losing a bit of the crunchy exterior, wrap it in foil and put it in a low oven (200 to 250°F ~ 95 to 120°C) until it's back up to the temperature to serve it at.
If you want it re-crisp the outside, you can either unwrap it as it's almost at the right internal temperature, and turn the heat up some ... or you can put it under the broiler (grill ... whatever you call heat from the top only)
Another option, if you're serving it au jus (in its drippings) is to heat up the drippings, then put slices in the juice to warm it up ... but at 18lbs, that might not work so well. (although it might be worth trying to heat it up sliced, rather than one giant roast, if the interior has cooled down significantly)
Also consider removing it from the oven before you typically would and how you pack it for transport.