car shaking at certain speeds

Hello, the most common thing that causes the car to shake when cruising between 40 and 60 miles per hour is when your wheels are out of balance. Typically this causes a rhythmic vibration felt in your steering wheel between 40-65 miles per hour. Seeing your local tire shop or repair shop that has tire equipment to have your wheels rebalanced should resolve your concern. Now, there are other possible causes -- such as an out of balance axle shaft, for example. And, if your shaking happens only during braking, then it could be related to brake rotors being out of round. Based on your question, I'd start with wheel balancing though. Good luck!