I own a 2010 Buick Lucerne with only 59k on the odometer. I just bought a new set of Goodyears, had them road force balanced and they said all looked good and the rims are good to, no bent rims.. But when I hit 55-60 mph the seat vibrates and then it smoothes out again when I reach 70. Amy ideas in this? I thought road force balanced was as good as it gets.