I need to buy a gas cap to go under my flip top fuel cap on my 1970 challenger. I see they sell a cap that is non vented and a locking cap vented. Do i need a vented cap ? or will the non vented cap be fine

I used the non vented cap on my 70 Challenger. There is a vent in the filler neck.

If your car is a California Car - it will have an N95 on the tag. If so you need ECS cap, if not you need a CAS cap.

Or another way to tell is whether you filler neck has small female or large female screw openings for the cap to lock into - small female is ECS as well....