There's an opening position "Fueling Opener(s)" that will fuel up cars used the previous day that were not checked.
There are 116 cars (92 in operation at one time) each with a 10 gallon tank (and the track is 2210 feet - 4/10 of a mile).
They run at 7.5 MPH.
Trip time + Dispatch time = 5.5 mins.
So, on a very busy 10 hour day, a vehicle can make 110 trips ~ 46 miles - on 10 gallons (and the cars do idle all day...).
I'm guessing the 10 gallons lasts all day (or most) and they can cycle in the extra cars if they run low (or fuel it up in the pit).