because my car accelerates quickly, but when it brakes it wants to continue advancing, it gives like jumps.

@percymarina
Try reducing the torque and or wheel friction values.

the friction of the wheels in which way I find them

Mudrunner editor folder / _mods / <your mod here> / classes / wheels / <your wheel name>.xml

XML file for the wheel in Media\classes\wheels.

<_templates Include="trucks" />
<TruckWheel>
    <WheelFriction _template="" />
</TruckWheel>

OR

<TruckWheel>
    <WheelFriction BodyFriction="" SubstanceFriction="" />
</TruckWheel>

BodyFriction value is for hard surfaces. SubstanceFriction value is for mud. _template is for using predefined WheelFriction value templates. Such as "Offroad", "Highway" or "Default". <_templates Include="trucks" /> line must also be used with _template.

Offroad: BodyFriction="1.0" SubstanceFriction="1.2"
Highway: BodyFriction="1.5" SubstanceFriction="0.5"
Default: BodyFriction="1.25" SubstanceFriction="0.75"