Hyundai - Excel :: Idle Would Go Up And Down When Turn On The Headlights

My first car was a 1986 Hyundai Excel. I spent about as much time fixing it as driving it, but it taught me how to work on cars, which has saved me a lot of money over the years. It had some quirks. When I turned on the headlights, the idle would go up and down. When I looked at the carburetor, I could even see the throttle open and close. It had one of those complicated feedback carburetors that I don't really understand. My question is about the transmission. It was a five-speed, and the clutch never slipped. When I got the car, the speed limit was 55. At that speed the engine would be somewhere over 3,000 RPM (I forget exactly).

Then a couple of years later the speed limit went up to 70, and at that speed the engine ran at 4,000 RPM. The strange thing is that the engine speed was the same in 4th and 5th. I have a book about cars that I got years ago, and it mentions something called a "fifth gear" that isn't an overdrive but "acts like an overdrive." What this means, but it seems to be what my Hyundai had. I found no difference in engine speed or performance. If I had to downshift to 4th to climb a hill, I'd immediately have to downshift to 3rd, because 4th made no difference. What that fifth gear was for? I also wouldn't mind knowing why the throttle opened and closed when the lights were on.