We have a fuel temperature gage reading tank temperature from nē1 main tank.
That tank is located on the wind above nē1 engine.
We can read the fuel temperature received by each separate engine as well.
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We are to keep the fuel 3ēC above fuel freezing temperature.
Jet A-1 freezes at -40ēC - That makes the minimum temperature to be 37ēC.
Our "book" says to use TAT in case the fuel temperature probe is INOP.
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Generally, the TAT is generally around -30ēC, give or take 5ēC.
Generally takes a few hours, for fuel to go down to that temperature.
We have fuel heat (upstream from FCU) which can be selected using bleed air.
Another way is to increase Mach number = increase rise = increase TAT.
Or... descent...
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Old PanAm trick of the trade - Keep 5,000 kg (10,000 lbs) of fuel in CWT until later.
Fuel in CWT (the belly near air packs) is much warmer.
If the fuel went very cold (often on 747SP) we could use warmer fuel from the CWT.
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Obviously I do not know how goes with a 777, but likely to be about same.
Happy contrails