I have a 1967 Olds 442 and a friend of mine was driving behind me noticed that when I accelerate gas was spewing out of tank... with non vented cap on. Any suggestions for fix. This is second cap I purchased that I thought fix the problem.

 

I merged your 2 threads. Are you sure it's the gas cap and not rotted vent hoses that run up behind the license plate?

 

It's definitely gas cap