The most common reasons for a steering wheel to shake at 70 miles per hour are uneven tire wear and a small bend in the wheel. The vibration is felt in the steering wheel as the harmonic frequency of the rotating tire exceeds the dampening capabilities of the car's suspension.
Tires are rotated, balanced and aligned when they are newly purchased. After that, tires are rotated at specific intervals to keep tread wear even, as the wear pattern is slightly different for each tire. Improperly balanced tires cause vibration at high speeds, as do tires that are out of alignment. When a steering wheel continues to shake after all tire-related issues are resolved, the car's suspension is typically the cause, according to tires and wheels expert Sean Phillips at About.com.