Driving 10km slower for gas mileage

For us the sweet spot is somewhere between 60 and 65 mph. We realize that this puts us outside the legal limit of 55 mph in some states and too slow in others that can do 75. However with car traffic and some truck traffic often moving in the 80mph range, the lane turbulence is scary even at 65. We were nearly wrecked in January when some yahoo moving at an estimated 90 mph tried to avoid slowing down for traffic in the opposite lane by squeezing between us and the vehicle to our left. Locked up all the brakes and had a serious white-knuckle event. ugh.
Bill