Folks... it does NOT matter what the power SOURCE is.
DC 12v, DC 18v, AC 110v, AC 220v... all of that matters not a whit. There are only two things that matter:
DC 5 volt is being fed to the phone. All else is irrelevant wherever it comes from. Hence, why voltage doesn't matter--the phone can only use DC 5 volt.
Amperage: this determine how much "juice" is being drawn. Now folks, the power is not "pushed" to the device. The device "draws" the power from the available power source. The phone will draw only up to 1 amp. It was designed that. So, if you plug the phone into a DC 5V/10Amp source, it will only draw 1amp. If you plug the phone into the iPad's wall charger which is a 5v/2a source, it will still only continue to draw 1 amp. However, if you plug the phone into your computer's USB port, USB ports only have .5 Amp of 5v DC current. As a result, the phone can only draw .5 amps (or 500 miliamps) since that is all that is available.
Back the original question: Can a car charger charge 3x faster than the wall charger? Impossible. The phone will only draw 1amp of power, it matters not a whit if more juice is available.