Interesting projecect.
My car is also a built for fun+economy, however I'm surprised you get a significantly worse fuel economy than I do with a significantly smaller engine.
I would suggest looking at your driving style and running higher pressures for normal driving than on the track.
I'm running Toyo Proxxes T1R tires on my car at the moment.
For road use I go with 3,0 bar front and 3,5 bar rear to reduce rolling resistance.
For fun use, I'll drop pressures to 1,8 and 2,0 bar.
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