Yeah, poor maintenance is a problem. Cars are so reliable today that people never give a tune-up a second thought. They generally don't need it, but things do go wrong from time to time. A bad oxygen sensor or partially clogged converter can have a significant effect on mileage, but until the idiot light goes on, people are oblivious to the situation. Of course, not everybody is like this, but there are plenty of people that won't even get the oil changed every now and then.
I track my mileage with every tankful, since it is a very good indicator of when things are working properly or when something isn't quite right under the hood.
I know folks who drive with tires at 10 psi. I know folks who don't bother with an oil change til their engine runs dry. And then they need more than an oil change. Their idea of preventive maintenance is waxing the car. Sigh.