what would cause rpm to drop after reaching 40 mph

My idea as well. I think that the transmission and you just don't realize it. Try this. When it happens, and there is no traffic around, stomp on the gas pedal all the way to the flood, and see if it shifts back into the so called passing gear. You can both hear it and feel it, when it does. If that is the case, and you don't actually slow down when this happens, then it is just how cars and trucks are designed. Just like on a 10 speed bike, when you get going faster, you shift into a different gear, and you can go faster with less effort, than if you tried to only stay in first gear. If you cannot figure it out, then if you have a check engine light on, go to an auto parts store, and ask them to come out and check and see if the car has any stored numbers that tell the computer what is going on. I do hope that one of these 2 answers help you, but if not, you should get it to a repair shop very soon, since that is a condition that could possibly cause an accident or burn up the catalytic converter, if it gets too hot.