Looking at the picture in your post it appears the focus point was on the guy in the back as the front bumper decals are more out of focus than the tank number. Your aperture at 5.5 is close to wide open at 300mm . If you could have stopped down the lens and lowered the speed proportionately you would have increased the dof to include the entire scene perhaps. Then you could tell better how well the lens was doing. I'm just on the laptop so take the comment with that in mind.