Depending on how unbalanced a wheel is it can vibrate at any speed. The balancers at my old shop spun at 40 MPH and 70 MPH, I believe. The larger the wheel the harder it is to balance correctly. Larger wheels also need balanced more often than smaller ones. I know this sounds crazy but trust me on this. I would suggest a rotate and balance every 4K-5K miles. If you are near a Discount tire or Tire America you can purchase lifetime rotation and balance for around $40. I highly suggest anyone to do this, it pays for itself after the first trip!