What muffler would be best to put on a G37x

 

Probably the phrase you're looking for is "axle back", as in the pipes and parts after the rear axle.

Tanabe makes a reasonable axle-back upgrade for the G37 Coupe and Sedan... different cars, different exhausts.

You didn't say which. Here are retail links to both:

https://www.z1motorsports.com/nfs/re...t-p-19909.html

https://www.z1motorsports.com/rear-s...t-p-13091.html