If your steering wheel begins to vibrate at speeds of 40 mph or more, your tire, wheel and possibly the brake assembly is out of balance. When this happens after miles of trouble free driving, one possible cause is that a wheel weight has come off the wheel since the tires were installed. This causes the weight of the tire and wheel to be unequal from one side to the other, and the result is vibration which is felt through the suspension of the vehicle and up through the steering column to your hands on the steering wheel. Losing a wheel weight is quite common and easily fixed, usually at no charge wherever you had the tire installed originally.
Another common cause of shaking are mechanical brake problems. If your steering wheel shakes while you are braking, then the problem is likely caused by "out of round" brake rotors. This vibration can also be felt through your brake pedal. Another common problem that can cause shaking and vibration is when a brake caliper sticks. This is when the brake pad does not fully retract after engagement with the drum or disk. This type of vibration is usually felt through the brake pedal or via the seats.
Another cause of a vibration at higher speeds is uneven tread-wear. A tire that has been on a vehicle for a year more, more start to wear more on one side of the tire. This is not the sign of a manufacturing defect. More likely, the cause is a slightly mis-mounted tire. It is possible that the wheel itself has a heavy spot, which is offset by a heavy spot in the tire which is positioned at 180 degrees. If the tires are not too badly worn, having your tire installer re-install and balance the tire can usually solve the problem.
A bad shake or vibration can impact the drivability and overall handling of the vehicle. A basic maintenance check by a qualified mechanic will usually identify the correct source of the problem. If it is a tire and wheel issue, reinstallation, balancing and rotation is the usual corrective action.