Gas cap

... or would you say in the UK (and nations with derivative English): petrol cap?

I was parked (on the right hand side of the road, where I live in Canada) waiting for my wife to return from the flower shop, and was going to snap a photo of her as she crossed the street, but as I pressed the shutter, a car passed in front and all I got was the gas cap and reflection of the buiding behind me. Oh well.