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According to people in this xda thread it's safe but I wanted to make sure. I'm assuming this is the same as charging an iPhone with an iPad charger which is safe to do and charges faster due to the higher amps. http://forum.xda-developers.com/showthread.php?t=1795225

I just got the Nexus 7 and it's charging a lot faster than my GS3 does when I use the GS3's charger.

I use my Mom's car charger all the time on my GS3.. she actually uses some mid-grade LG phone. Works fine with me. It's all microUSB and I'm assuming it pulls the exact same wattage. You should be fine.
 
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