[s-cars] rusted gas cap on my S6

[Jake Gaty]

I filled up the other night and the gas station attendant came up to my window and complimented me on my car, but then he showed me my gas cap and I saw there was a lot of rust on it, and that some of the green plastic was cracked off.

Slight backtrack - bear with me - When I've filled my tank up from completely empty to full, The needle is all the way up past the F and it stays like that for at least 30-50 miles before it starts to make its way lower. My issue is when I get in my car the next day the needle goes to just above 3/4... where is all of that extra gas going? 

is the rusted cap letting gasoline evoporate out of my tank? and where can i find a replacement?