Is it possible in any way to buy a mpg thing that tells you the mpg for your car ?

Can I ask you something?

I drove at 60 mph driving 180 miles on motorway from full tank, afterwards I estimate it will take £13 to refill to brim ( 13 litres) so this = 62 mpg
I drove another 50 miles, this time around town and the gauge hit half way on fuel. it takes 22 litres from half way to brim, so I calculated again but did 230 miles from 22 litres and it is only showing 44mpg
Now if I continue driving urban until fuel light comes on it is usually another 200 miles from half way to light on. so that will be approx. 430 miles from full tank. working that out it comes to around 43 mpg from full tank, which is only 3 mpg more than I usually get from urban alone despite that long motorway trip driving 180 miles at 60 mph

How is it that the first 180 miles saw me get 62mpg ( 180 miles from £13) , then if I drive 50 miles more it drops from 62 to 44 mpg and if I wait till fuel light comes on then the mpg drops to 45mpg ?

In my car, the fuel light comes on (with the gauge hitting the empty bit) when I still have ~100km of fuel.

Regardless of your observations, the fuel level gauge is not accurate enough to work out mpg. There is no point trying. The best you can do is work out your average mpg between fuel-ups. This can be done very accurately.

Can I ask you a question ?

Is it normal for water to travel through overflow pipe into coolant tank ?

That is an odd change of subject. Surely though, if it is an overflow pipe, the water should flow out of the system? Has the system been pressure tested?