The spare wheel was probably with the car - if you know where to look.
Something that bothered me a bit was that Chris, Daryl and Brad made a huge boo-boo in this movie, - When Chris' Mom's Buick has a blow-out on the Feewayway and when they look for the spare wheel they think its gone, - they just looked in the wrong place! Chris flips-up what looks like the boot/trunk floor, (where most cars DO have the spare) But on B-Body wagons, the Buick, Olds, Chevrolet and Pontiac have the spare in an upright-well at the side of the car in a storage compartment next the boot floor.
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