i cant answer all the questions, but i can give some personal experience. i have a 1998 subaru outback wagon, i replaced the rear speakers with 6.5 in focal with a solid cone in the middle, the stock radio has enough juice to power those speakers plus the front. it really all depends on what you...
if the battery and alternator are good, and its not the starter, what else might it be? does the engine crank over? if so and its still not starting, try taking some carburetor cleaner and spraying that in the intake, right where the intake manifold is. if the truck somewhat sputters, then we...
your overdrive in your hyundai is built right into your regular drive, so there is no way to put it in overdrive, it just naturally does it. so there is nothing you really need to worry about it, also doing highway driving at speeds of about 65-75 get really good mpg results