ffmpeg default output frame rate

Question

What is the default output frame rate chosen by ffmpeg to encode MP4s?
Where is this specified on the man page?

My input video stats are:

• r_frame_rate=120/1
• avg_frame_rate=31230000/1042111
• duration=11.579011
• nb_frames=347

After (demuxing, decoding) splitting (rescaling, encoding and muxing) the video stream I end up with has the following stats:

• r_frame_rate=120/1
• avg_frame_rate=120/1
• duration=9.575000
• nb_frames=1149

I started with 347 frames and ended up with 1149 (should have been 287). The input average fps was 29.97 and now I end up with 120... which was the max fps of the input video. So, I imagine ffmpeg picks the highest one from the input... but is this specified anywhere? How does one go simply matching the input fps? And why does I have a variable fps to begin with?

Hmm... too may questions I guess. Still, is this simply a consequence of this paragraph?

STREAM SELECTION                                                 
       By default, ffmpeg includes only one stream of each type  
       (video, audio, subtitle) present in the input files and   
       adds them to each output file.  It picks the "best" of    
       each based upon the following criteria: for video, it is  
       the stream with the highest resolution, for audio, it is  
       the stream with the most channels, for subtitles, it is   
       the first subtitle stream. In the case where several      
       streams of the same type rate equally, the stream with the
       lowest index is chosen.                                   

Answer 1

r_frame_rate is "the lowest framerate with which all timestamps can be represented accurately (it is the least common multiple of all framerates in the stream)."

avg_frame_rate is just that: total duration / total # of frames

You can just specify -r 30000/1001 to maintain the average rate (near-abouts). You don't specify which format you're outputting to, but for MP4, ffmpeg defaults to constant-frame rate, where it picks r_frame_rate as the value. It will then duplicate or drop frames to keep that rate. Use -vsync vfr to keep the variable rate.