He felt about this and touched something else.

He felt along it; it was smooth and continuous.
And then he knew, and he experienced infinite relief.

His hand was upon the spare tire on the rear of the car.

The air was slowly escaping in irregular jerks from the valve of this tire, making that low sound, now hardly audible, now clearer and steadier, that escaping air will sometimes cause when passing through a leaky valve.

The darkness and Pee-wee's own thumping heart had contributed to the horrible illusion and he smiled in the utter relief which he experienced by the discovery.
But one other discovery he had made also which gave him an inspiration and made him feel foolish that he had not had the inspiration before.
The little round thing that he had felt in about the center of the tire was the red tail light of the car; he realized that now.