your 83 300D can do 170km/hr. Or about 110MPH. Ive had mine 1 time going the full 110 with the help of a hill. If everything is in tune, valves adjusted, turbo working, and some redline additive in the tank, yours should do 100+ as well.
Your tach is behaving this way because you need a new sending unit, $80. U can replace it yourself. Happy driving!