Extract data from json by using a subscription

Hi,

I'm currently using MQTT to send sensor data to the thingworx platform. The data is in json format and looks like this: 

{"tire_pressure":4.7,"speed":55,"timestamp":"2020-05-07 11:25:19.971","status":"success","vehicle":"car"}

In the MQTT master template I created a property called Speedometer which receives this json data. Now, I'm trying to extract the data by using another property (called Speed, base type: int) which subscribes to the Speedometer. The subscription looks like this: 

me.Speed = me.Speedometer.speed;

But the property Speed doesn't change it's value. Do you know how to solve this?