40

This number is a composite.

The NASCAR Cup Series typically uses a 40-car field. Coincidentally, there are 40 possible two-digit endings of primes: 01, 03, 07, 09, 11, 13, 17, 19, 21, 23, 27, 29, 31, 33, 37, 39, 41, 43, 47, 49, 51, 53, 57, 59, 61, 63, 67, 69, 71, 73, 77, 79, 81, 83, 87, 89, 91, 93, 97, and 99. Consider a "prime race" where we count how many primes there are, up to some number of primes, having each possible two-digit ending. After the first 1,000 primes, "Car Number 57" is leading with a count of 29. The leader after 10³, 10⁴, ... , 10¹¹ primes is 57, 93, 57, 77, 91, 17, 47, 51, and 31. The leader after a trillion (10¹²) primes is unknown, therefore "Gentlemen, start your engines!" [Honaker]