MK2 Supra. Digital Dash Problem. Please Help?

I just bought a 1985 Toyota supra today and i didn't check if the speedo worked or not. so i finally got it home after i long trip, and drove it up the street and i noticed that the speedo wont say how fast I'm going. it just says 0 all the time, maybe sometimes it will read 1-4km's per hour when I'm going slow but otherwise it just reads 0. Please i need help.