^^Holy crap that's horrible fuel mileage. I've only ever heard of two sizes of tank for the e30 55 litres and 60 litres. My M20 goes at least 600km per tank usually more like 650 to 700km per tank. Which is 8 to 8.5 litres for every hundred km.
In regards to the a/c, I think you're right, too much pressure can cause that situation. I had too much freon and the compressor and it was acting funny. Lowered the pressure and acting fine now.
BMW 325I E30 Chumpcar Race Car
BMW 540iT M-Sport (New Family Hauler)
BMW 135i Msport Daily/track toy