So, you're a multi-millionaire college football coach. You can drive any car you want. Literally any car. See car, want car, buy car.
If you're Gus Malzahn, maybe that's a $140,000 BMW sports car.
If you're Nick Saban, you drive a Mercedes because you have a stake in your own Mercedes dealership.
Though you can afford any car you'd like, chances are you can drive for free because your contract calls for the use of two luxury vehicles at the university's expense.
Of, if your Purdue's Jeff Brohm, you can drive your old 2004 Honda Accord.
I would say this is a metaphor for Brohm's offensive philosophy, but I think he just really likes driving a cheap car.