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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?

Son Jerm
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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.

stretch
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  • So it is not possible to have a negative fuel efficiency? – Son Jerm May 07 '17 at 12:34
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    @Son Jerm No, it's not. But it was a gag in an American film in the 1970s (Pete 'n' Tillie). It had two of the main characters adding gas to the tank of a Volkswagen beetle at night, while its owner was asleep. He was a gas mileage bore - continually bragging about his car's economy. Eventually the car seemed to reach that negative efficiency. – stretch May 07 '17 at 12:49