Gas is ~3.50/gal. Assuming the vehicles get a horrendous 10 miles/gallon (for a fuel cost of $0.35/mile), and the drivers are paid minimum wage (7.25 + employer costs = ~8.00), the cars would have to be driven 22.8 miles in a hour before fuel costs exceed labor costs.
Of course, the vehicles are probably getting twice the MPG, and the drivers are getting paid more, so the number of miles is probably much higher.
Of course, the vehicles are probably getting twice the MPG, and the drivers are getting paid more, so the number of miles is probably much higher.