M62 4.4 fuel consumption gone down

[Themoog]

Thinking caps on.. My 4.4 M62tu has gone from 19mpg average to 16mpg average in the last few months. I've also noticed that the temperature gauge sits slightly below vertical most of the time.
Is my motor running too cool?
I know the guage is buffered between 75°c and 110°c when needle is vertical but my diagnostic shows the engine temp never going over 85 so is the fuel system stuck in a closed loop and over fueling because it thinks the engine is not up to temperature? Is that how it works?
Car runs fine otherwise.
I've recently had a new set of tyres too but the car rolls easier and the steering is lighter so I don't think think it's them causing the poor MPG.
What do you think?