|MadSci Network: Zoology|
According to this site on the web: Speed of a Snail or Slug , "Snails and slugs travel at speeds that vary from slow (0.013 m/s) to very slow (0.0028 m/s)"
That range of speeds translates to between .006 to .03 miles per hour ...very very slow. I guess it would be better to make that feet per hour rather than miles per hour: slugs can move at 30 to 150 feet per hour.
Try the links in the MadSci Library for more information on Zoology.