Took a 600 mile round trip to Cortland NY ( yesterday) to pick up my daughter and 3 friends from camp . Got 18.6 mpg going up at 75 to 80 mph. Got 18 coming home averaging 80 with some traffic during rush hour at the bridges and Long Island highways . 4 extra people at about 500 pounds and about 500 pounds of stuff so, (1000 lbs total).I did notice a shaking / vibration at 80 to 85 (noticeable ) , barely detectable at 75 but still there , and gone at 70 or below . Vibration there under load or not and in neutral or not and ( sometimes )seemed to not be there . I'm thinking tire out of balance or alignment , but which one ? It sometimes did seem to lessen with a slight turn of the wheel at those speeds but was still there and or came right back. ...... I've got 36k on the truck with the original crappy pirellis with decent tread life on em........ What you guys think ?