My car has started shaking when going over 60 mph but under 70 mph. I am not sure what the cause of this is.
Shaking at higher speeds often indicates that one or more wheels is out of balance. These wheel situations may not cause vibration at lower speeds but start at a specific speed. Often the condition is easily repeated and demonstrated for a mechanic. The vibration may stop at some higher speed, or it may continue. This is due to the specific dynamic nature of a given wheel’s out-of-balance cause.
The solution for this condition is to have the wheels balanced on a high-speed balancing machine off the vehicle.
Other causes of vibration are spinning parts that reach a resonance and have enough rotational mass to cause vibration at higher speeds. This includes bent or loose engine pulleys, lose belts.
Try to judge if the vibration seems to be from the engine compartment or more likely from a wheel or wheels.