I think that this is untrue. According to the MP3 frame header "spec" on id3.org, MP3 frames seem to have the following fields in their headers:
  • Sync
  • mpeg-1/mpeg-2
  • Layer (mp2/mp3)
  • Checksum existence flag
  • Bitrate
  • Frequency
  • Padding byte existence flag
  • Private bit (?)
  • Stereo/Mono Mode
  • Mode extension (?)
  • Copyright bit
  • Original media bit
  • Emphasis
  • Optional checksum
None of these fields seems to be for volume change. Looking at the mp3gain source, it looks like it slaps the gain info in the Xing/Lame Info header/tag.

That being said, is the car player responding to mp3gain changes? How about the replaygain-suggested RGAD ID3v2 frame, if it were set?
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Bitt Faulk