Plot hole: When the gang goes to retrieve all the cars from the guy living in the desert they determine they have enough cars for a a one car safety limit. As they discuss this Jim is measuring the length of the blue car Jeff will be driving but the license plate of that car is never taken into account when being measured, so they should have already known that even with enough cars in their possession they didn't have enough cars to make a mile.
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Suggested correction: They had a one car 'cushion", but it wasn't the blue Mercury, it was the car on the trailer the Jeep was towing. They had just enough with the Mercury, but he crashed it coming into the parking lot which shortened it ; the plate falling down added the extra inches.
