A foot means that you can reach out and touch the truck. A foot also would mean that the truck as not approaching the centerline, but that it had gone significant over.
The car is going to stay in the middle of the lane unless it is actively trying to avoid something. Most drivers, tend to shy away from trucks and move away from them. This is actually a dangerous behavior, because they have decreased the margin on the other side of the car.
I had a car recently that was passing a truck in this same situation, but was so hesitant about being close to the truck that they went over the lane to the left of them.
The Model S is 7.2 feet wide, the average interstate lane is 12 feet, although some roads may get down to 10 feet. At that point, the Model S, between the lines will have 1.4 feet on either side.