I've had my '16 6 GT for about a month now and have settled into it nicely. I had to go out of town and rented a new Nissan Altima for a week, put over 300 miles on it. Here's my impressions.
Handling of the 6 is worlds better than the Altima, so is the ride comfort and isolation from bumps. Big worry I had with the 6 GT was road, wind and engine noise, as there were a lot of quips criticizing the 6 for this, particularly with the 19 inch wheels. I have to say the 6 is quieter than the Altima, especially the engine noise, and road noise is more noticeable in the Altima. I was surprised. Not to say its not a nice car, but the interior is not up to the Accord and the 6.
I can't understand why they sell more Altimas than the 6. Maybe less dealers?
I, too, was initially concerned about engine, wind and tire noise on my 2016 GT. Happy to say that I am pleasantly surprised. But I also quickly swapped out the OE Dunlop tires for 19' Michelin Pilot Sport AS3 tires and they vastly improved ride, comfort and handling. (Spoiler alert: I work for Michelin.)
As for why Altima sells more, it's probably advertising, dealer locations and comfort with the Nissan brand. Mazda has a bit of maverick in its DNA and so it isn't as mainstream as Datsun?, whoops, Nissan! My 2 cents...
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