Measuring speed and altitude from personal phone midflight Given that there is no need now for one to swith off their cellphone midflight, inflight wifi is now  a norm and common, Is it possible for me to measure the velocity and altitude of an aircraft using my personal car GPS speed indicator or cellphone app in the case of speed and altitude as a passenger?
Depending on the reliability of the sensors in your phone, yes that sounds completely possible. Most modern aircraft use GNSS for navigation, and your phone relies on the exact same technology.

Be aware that you will get the ground speed - see this question for further details: Why is there a difference between GPS Speed and Indicator speed?