I have found an expression that models the fuel efficiency of a truck and the expression is $(7-0.1(x-110)) km/L$.
Where $x$ is the speed of the truck. If fuel efficiency, in this case, means the distance travelled in km with the consumption of a litre of fuel what would negative fuel efficiency values mean. For example.
When $x=250$
$fe=-7km/L$ what does this mean? Is it even possible to attain a negative fuel efficiency value in real life (i don't think so), if no then does that mean that the truck would not be able to reach such speeds?
It's a straight-line approximation to a curved relation between fuel economy and speed in the range of normal highway speeds. It clearly doesn't work for slow or very fast speeds. It also wouldn't work for varying speeds, particularly stop and go traffic.
There's a singularity at 180 km/hr, beyond which the truck stops using and starts producing fuel, according to the expression: that's the meaning of negative efficiency. Of course the efficiency decreases as speed increases, but it never gets very close to 0.