You don’t say what tyres you have fitted - another Roadster owner will be along in a minute with much more precise advice.
I run our 2014 4/4 at 24psi on 165/80/15 Vredestein T-Trac 2 tyres.
I used to run the OE Continentals at 22psi. The 24 psi was recommended by the chap that fitted the tyres (Paul Harris) near the Belfry - sadly retired. He arrived at 24psi by looking at the side walls and inflating the tyres a little at a time until, to his experienced eye, they looked right.
This is a long winded way of saying that - you need them to be a lot lower but the actual value will be a matter of trial and error.
In your shoes, I would try 24psi all round next and see how the car drives.
As regards the fuel gauge, most people rely on the trip mileage. The gauge is notoriously vague.
