Cylinder miss, broken motor mount, or vacuum leak.
might have misaligned or loose wheels
the cuase of this is ussaully worped rotars
Could be a problem with your torque converter
depending on what car it is. usually 30 or 35mph.
Cylinder misfire. Possibly failed or failing engine mounts.
The engine of the car may need to be cleaned or your oil may need to be changed.
Normally this is caused by a tire out of balance or a bent wheel.
have front suspension checked
Is that you? My long ago Profesor, 35mph- 1mph/60min=25
wheels need balancing, sometimes shake is only evident at high speed
i would really like to know myself