Speed data logging using arduino and GPS module
According to the NeoGPS documentation for the gps_fix class, it looks like .lat and .lon are no longer part of the gps_fix class.
It looks like instead you will have to use .latitudeL and .longitudeL for high precision (Long Integer scaled by 10,000,000) GPS coordinates.
The solution would be to replace fix.lat in your code with fix.latitudeL and fix.lon with fix.longitudeL.