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I notice it completely mutes the audio where as Apple Maps merges them. I think i prefer the way apple maps does it.
 
You also can't search for POI's along the route while navigating like you can with the traditional app. I'm sure that's a limitation of apple's extra cautious implementation of carplay, but it's still annoying that the carplay version isn't as full featured as just using google maps on your phone.
 
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