Avconv ac3 to aac or mp3 is not synced with video

I'm trying to convert a MPEG2 file to H264/AAC.

No problem about video, but when I convert Audio: ac3, 48000 Hz, stereo, s16, 255 kb/s to MP3 or AAC, with any bitrate, the sound is not synced with the video. The sound is in advanced to video when I play it. How to fix it please ?

More information : the source video is 1h30 long and 3,4 Gio.

To sync the audio stream with video time stamps we may use the -async option:

-async samples_per_second
Audio sync method. "Stretches/squeezes" the audio stream to match the timestamps, the parameter is the maximum samples per second by which the audio is changed. -async 1 is a special case where only the start of the audio stream is corrected without any later correction. This option has been deprecated. Use the asyncts audio filter instead.
^{Libav documentation}

Begin to try with samples per second matching your framerate and decrease until your audio is in sync:

avconv -async 30 -i <infile> <encoding options> -r 30 <outfile>