56 truck speedo ,,

[JumboAMONGus]

hello , i have a 1956 chevy 3100 with original 3 speed , and rear end , the transmission was rebuilt years ago , works fine ,, but the speedometer only works in 1st and 2nd gear , once you put it in 3rd it drops to zero no matter how fast you are going ? the speedo gears in tranny are fine also is there something in the transmission shaft that engages when shifted to 3rd any info or a diagram , my manual does not show thank you ,