It's one of those questions you've probably never thought to ask, but the second you hear it, it's stuck in your head. Automakers have produced cars with three, four, five, six, eight, 10, 12, and 16 ...

Over time and miles, your car might start to slow down. It might not accelerate as quickly as it once did; the engine may not run as smoothly as you remember, or the fuel economy may dip—but ...