Like most, understand the car buying and as long as the economy is reasonable its not something fixate on - I drive, I fill up, I drive etc,
But the R and now the TCR have been pleasant surprises in the economy department, particularly when the performance need is completely secondary and I just want to crunch some long miles, the car does this 40 MPG trick

As a rule had to roughly fill the R up between 270 - 300 miles on my normal completely mixed driving of short/medium/sometimes long
Not counting the really short runs, on any next town journey lets say, the economy has not dropped below 30 and you regularly see it in the 32-34 range on screen.
As finding the DSG completely usable in S, my driving mode is individual with everything set to Sport except the suspension which is set at Normal and the Engine Sound which is set to ECO
Not changed my driving at all, but the TCR is averaging about 350 miles before have to fill up, and if both the previous GTD and R are anything to go by, this economy will improve.
So probably 4-6 miles a gallon more economy than the R
My GTD at end, where was not doing the long miles anymore, slipped to around 37 MPG due to these short and medium runs out.
Given the TCR is only 10 BHP short of the R, the gain in economy must be all due to weight and maybe the 7th gear of the MK 7.5
As ordered the TCR pre MY20 ownership is cheaper than the R and fuel stops are less frequent,

One thing with fuel that is different is the how many miles left calculation when the refuel light comes on, it counts down in real time! 30 miles is really 30 miles, where in the R if you took really easy it seemed to stay on say the 25 miles remaining figure for 10 miles - As soon as light comes on now I go and look to fuel up.