Standard Disclaimer: The only thing I know about 73s is, um... OK. I know nothing about 73s...
However, as I do whenever I have a 737 systems question, I consulted
http://www.b737.org.uk/index.htm which told me:
Fuel temp is taken from main tank 1 because this will be the coldest as it has less heating from the smaller hydraulic system A.
Thoughts?
Dave