Our model of truck is new and we haven't completely calibrated the torque control mode, this is why you get such unrealistic behavior.
In order to fix the issue you simply have to edit it's PROTO file (located in WEBOTS_HOME/projects/vehicles/protos/generic/Truck.proto).
You simply need to add the missing field (all the fields are documented here: https://www.cyberbotics.com/doc/automobile/car).
Basically, the two interesting fields to add are:
- engineFunctionCoefficients: defines the torque function of the engine.
- gearRatio: defines the coefficients and number of gears.
If you manage to get realistic value do not hesitate to send them to us so that we can fix the issue for the next release of Webots. And if you can't do not hesitate to ask us for help.