To force the frame rate of the output file to 24 fps: ffmpeg -i input.avi -r 24 output.avi
To force the frame rate of the input file (valid for raw formats only) to 1 fps and the frame rate of the output file to 24 fps: ffmpeg -r 1 -i input.m2v -r 24 output.avi
From what i read, by default it’s supposed to have the same speed, the -r option only changing the sampling rate.
Have you tried another image viewer? Some may have trouble rendering animated gifs.
Otherwise maybe look at imagemagick, i remember it having options for managing the output frame rate.