What causes a car to shake at 55 mph?

A car may shake at 55 mph due to several factors, including unbalanced tires, misalignment, or worn suspension components. Uneven tire wear or damage can also lead to vibrations at higher speeds. Additionally, issues with the drivetrain, such as a failing driveshaft or worn-out engine mounts, can contribute to the shaking sensation. It's advisable to have the vehicle inspected by a professional to identify and resolve the issue.