Solved

Car compatability

How does car compatability work and what influences it is there something I can change to improve it

It depends on the tracks Andrew,
for example Monaco requires higher Downforce and Handling of your car while Monza/Italy requires more Acceleration/Top speed. The length of the straights in the circuit defines how much fuel consumption do you need (that's why China is one of the most fuel consuming tracks). a good Tire Wear rate will help you in tracks where the surface is so abrasive, like Spain.

The bottom line, it all depends on the characteristics of the track so you have to distribute your design points accordingly

Thanks Kevin,
I assumed that Andrew was asking about the "car compatibility" rating that is provided in the "Next Race" screen and I answered accordingly. Now that rating gets affected in a way by how you distribute your Design Points. I had my car compatibility ranking for Monaco changed from 2.5 to 3 stars just because I put all the DP that I earned in Spain into Downforce improvement. I gained even more for Germany.
Of course the game tries to emulate the actual characteristics of the actual tracks as much as possible and of course it doesn't work out 100% but the tracks that I have raced in so far seems very close to the actual ones.
I'm guessing you have maxed out the Fuel Efficiency so all tracks seem to have the same Fuel usage but I can assure you it's not the same for me (lvl 6 manager). Bahrain/China can not be compared to Monaco in terms of Fuel used per lap. And even though push level changes the consumption levels it still depends on the tracks and the extent of full-throttle pushing within the lap.
Finally according to Jamie's Amateur Guide and to my practice/test laps no track wears the tires more than Turkey followed by Spain (Bahrain & China are pretty close but not more than Spain).

I don't think the length of straight are the number one factor of fuel consumption. The amount of hard acceleration are in theory requires more fuels. 

Would be about right, in real wheel-spin in corner exits eats more fuel along with getting the hammer down early. Then pinging the rev limiter down the straight saves fuel as well as lifting off early and coasting into the corner. So in theory you are getting back what you lost and fuel consumption is steady unless you mess up somewhere in the lap to use more fuel.

Yes it's car compatability for each race I notice the stars change depending on the race

Stars are meaningless!
Take Monaco for example to get a high star rating you need Cooling & Reliability but they do nothing to car.

Thanks for the response
The cooling and reliability are not as important as say acceleration and braking and they are the last items I improve on my car but they do reduce engine and parts wear
it noticed it end of last season when the car is nearly fully developed I had a 4-5 star rating on the tracks
this year I have a 4 star rating and wondered what I could do to improve it but I guess the answer is you cant unless you want to waste design points on cooling and reliability before the main items