Some people say that rested XP adds 50%. In my experience, it triples your XP…at least for killing mobs, solo, in Classic…at least in the 20-40 range.
I tested this yesterday. At level 27, my rogue was killing level 22-26 bears in Hillsbrad and checking the XP after each kill.
When I was rested, each 23 bear kill gave me “230 XP (+115 rested XP)”
When I ran out of rested, each 23 bear kill gave me “115 XP”
I’ve seen this on other characters (in the 20-40 range) when rested XP ran out. The per-kill XP drops to roughly 1/3 of what it was rested.