The Project Pad

I have a quick question about our gas tanks on the C4 S cars. I don’t know if all C4 cars are the same but I have read several times that our gas tanks are 21 gallons. I have experienced this in the short time I had my first S4. I filled it once as it was almost empty when I bought it and got either 20 or 21 gallons. But when I bought my second one, I filled it and I was only able to get 16-17 gallons and that is with the car almost on “E”. I think it was like right on “E” when I bought it and got 17. I have only drove my car about three times in the last three weeks or so now, and when I filled it today, it was almost empty I only got about 16. Is there something maybe with my charcoal canister or anything maybe I could look into? The fuel gauge does rear full, sometimes less than a needle off from full. Just wondering if anyone else has experienced this or has some insight.

When I owned an C4 S6 Avant it held 18 gallons of fuel. When the fuel gauge got into the red it had 2 gallons of fuel left. The last and only time I replaced a fuel pump on a C4 S car there was some rust on the floaty thing-a-mahicky that went to some electrical stuff. (AKA the fuel gauge sender???) It’s been awhile but think I remember the fuel gauge needle not quite on the full mark on my old S6. Hope this helps.