Don't buy a car mat. Unless your child has a car themed room, then it would be really cute as a rug. But if you're buying it to engage your child in playing with cars, skip it. My kids had one, and they never used it in the way it was intended. My classroom had one when I was working in child care, and ditto. Save your money. Instead do this:
That is masking tape on the crappy carpet in our playroom. Yes, I have the advantage of having crappy carpet. This is cheap, it's temporary, and masking tape won't leave residue behind. Plus you can engage your child in laying out the roads, especially if they're older. I did most of the taping, but my kids had a lot of feedback- it's thanks to them that a corner of the room consists of a "race track." Then, when we're over it, we rip it up and toss it. Unlike the car rug that they ignored, which is now a grease catcher in dad's garage.