digital dash....

Hello all , newbie to this forum and a newbie to the Lebaron convertible, that I just bought cheap.

Now my question is , that when the vehicle is cold and I start driving the the digital dash hardly moves , I might be going 50 km but it only registers 16 km. As the car warms up then slowly the speedometer registers higher say If I'm going about 80km the dash would register 50, and so on. does anyone else have/had this? is there some sort of senser somwhere??

Thanks.