I have a flac library with both track and album replaygain values set on all tracks. I'm writing a script to mirror the library in mp3 in a parallel directory. Part of what the script does is takes the vorbis tags in the flac files and copies/translates them to their equivalent id3 tags for the newly create mp3 file. Are the replaygain values from the flac files directly copyable to the mp3 files? I have no need (I don't think) for mp3gain, as my mp3 players have no problem understanding replaygain tags.

edit: clarification

They'll be very slightly different. Unless you're incredibly sensitive to that sort of thing, I'd just copy them.

it depends on the mp3 encoder..

if you use LAME CBR make sure to add "--scale 1" to the commandline (not for VBR)

all Fraunhofer encoders I used so far decrease the volume of the encoded track a little, like 0.2 to 0.4 dB

Awhile back I noticed that Lame cbr and abr (--preset) encodes lost some volume compared to vbr encodes, and tested to find out the differences. The procedure is detailed in this post. Here's the stuff from that post that applies to the present discussion:

General findings:
* The amount of volume lost wasn't exact at a given encoder setting - it varied within a range of +/-5 dB, usually. The range (and thus the standard deviation) was tigher at higher bitrates.
* Higher bitrates had less volume loss.
* Variation in dB loss from song to song (in the second group, where the song volumes varied) did not appear to be correlated with volume. So, a wav file with a volume of 98 dB should lose the same amount of volume upon encoding with --preset 128 as a wav file with a volume of 92 dB.
* Using --scale 1 did not guarantee exactly equivalent volume; the songs still varied within a range, but given a sufficiently large sample the mean deviation was 0 dB.

Specific findings - volume loss at given settings:
--preset (abr) 128 - lost 0.58 dB avg, range 0.54 to 0.65 dB
--preset cbr 128 - lost 0.60 dB avg
--preset 128 --scale 1 - lost 0.00 dB avg, range -0.08 to +0.03
--preset 136 -b 128 - lost 0.61 dB avg, range 0.54 to 0.69 dB
--preset 145 - lost 0.43 dB, range 0.41 to 0.47 dB
--preset 160 - lost 0.44 dB, range 0.42 to 0.47 dB
--preset cbr 160 - lost 0.44 dB, range 0.41 to 0.46 dB
--preset 176 - lost 0.26 dB, range 0.21 to 0.34 dB
--preset 192 - lost 0.25 dB, range 0.23 to 0.30 dB
--preset cbr 192 - lost 0.26 dB, range 0.22 to 0.31

Note that this was done over a year ago, and I think I was using Lame 3.92 or 3.95. However, I have no reason to believe things would be different with 3.90.3 or 3.96.1 or 3.97alphas.

edit: table formatting