Google Maps is accurate for measuring distances. If it wasn’t, we would know since billions of trips rely on its accuracy for driving directions.
To measure the entire circumference of a line of latitude you can measure a section of the line of longitude and multiply by the appropriate factor to get the entire circumference.
Below we see that from -39, -73.3163 to -39, -62.312346 is 950.32km according to Google Maps.
This is a difference of 11.003954° longitude. This is 3.05665% of the circumference of the line of latitude.
To get the entire length of the line of longitude:
360° / circumference = 11.003954° / 950.32km
( 360° * 950.32 ) / 11.003954° = c
c = 31,101.38 km
The circumference of the equator is 60 Nautical miles * 360 = 21,600 Nautical miles = 40,003.2 km
The circumference of 39°S Latitude is less than the equator. This is mandatory on a globe.