Most car chargers lie about their ratings so that could be one problem, the other, is most phones see car chargers as a usb port charger instead of an ac charger so the rate of charge is automatically reduced to usb charging. Somewhere on here was a thread on how to modify a car charger so the phone would see it as an ac wall charger and allow it full output. I believe you have to open up the charger and jump a wire over. Search on here or google it and you should find it.
Phones & Tablets: Galaxy S4, Galaxy Note 8", Evo 4g LTE, Evo 3D, Acer Iconia A500, Samsung Galaxy Tab 2 10.1, Ipad 2, Nabi 2,
Rom: Rooted Stock Odexed, Hotspot Mod, Xposed, Wanum