Why does my car shake at highway speeds?

The most common cause is tire imbalance, and the symptom is characteristic: a vibration in the steering wheel that appears around 50 to 60 miles per hour, may smooth out at higher speeds, and is present whether accelerating or coasting. Wheel weights fall off, tires wear unevenly, and mud or snow packed in a wheel throws it off. Balancing costs little and is the first thing to check. A bent wheel from a pothole, or a tire with a separated belt or a bulge, produces a similar vibration and is found during the same inspection, and a separated tire is a safety issue rather than an annoyance.

Where you feel it is diagnostic. Steering wheel vibration generally means the front tires or wheels. Vibration felt through the seat or floor points to the rear tires, or on rear-wheel and all-wheel-drive vehicles to a driveshaft, universal joint, or carrier bearing. Vibration that appears only under braking is warped or unevenly worn brake rotors, felt as pulsing in the pedal and shaking in the wheel, and is repaired by resurfacing or replacing the rotors. Vibration under acceleration that eases when you lift off suggests a failing CV joint or motor mount.

Worn suspension and steering components, including tie rod ends, ball joints, and wheel bearings, can cause vibration and are also the parts whose failure is dangerous, so a shake that persists after balancing warrants an inspection rather than tolerance. A humming that rises with speed and changes when you turn is the classic sign of a failing wheel bearing. Whatever the cause, vibration accelerates wear on everything connected to it, which is the practical argument for fixing the cheap version before it becomes the expensive one.