Tropopause temperatures & fuel efficency
 

How does tropopause temperature effect fuel efficency at flight levels? All other things being equal, would ISA -10°C or ISA +10°C be better for fuel efficency?

In addition, if ISA +10°C causes less fuel burn per hour, would it also cause a slower TAS because the air is less dense, thus potentially cutting back on any lb/hr increase?

Greetings

TAS increases 1 kt for each 1 deg Delta ISA so ISA + 10 would give TAS + 10 Kts, fuel bun increases as well, depending on engines, I think the average is 1% to 0.5% FF per deg Delta ISA.
At Tropopause delta ISA equals ZERO so no increase in TAS, :}
So what happens to the FF :E