[BroZeus]

Well that's because you are not taking current speed of vehicle into account.
EDIT:
Well first take out current speed add that speed to "speed" variable then multiply it like this -

PHP код:

stock GetVehicleSpeed(vehicleid)//to take out vehicle speed
{
    new Float:Vx, Float:Vy, Float:Vz;
    GetVehicleVelocity(vehicleid, Vx, Vy, Vz);
    new Float:rtn;
    rtn = floatsqroot(floatpower(Vx*100,2) + floatpower(Vy*100,2));
    rtn = floatsqroot(floatpower(rtn,2) + floatpower(Vz*100,2));
    return floatround(rtn);
}
//now in 2nd method like this -
 new
            Float: Angle,
        Float: AngleX,
        Float: AngleY,
                Float: VelocityX,
                Float: VelocityY,
                Float: VelocityZ,
                Float:c_speed = GetVehicleSpeed(vehicleid);//current speed
            ;
            GetVehicleVelocity(vehicleid, VelocityX, VelocityY, VelocityZ);
            GetVehicleZAngle(vehicleid, Angle);
            AngleX = floatsin(-Angle, degrees);
            AngleY = floatcos(-Angle, degrees);
            SetVehicleVelocity(vehicleid, AngleX * (speed+c_speed), AngleY * (speed+c_speed), VelocityZ);//note change in this line
        }