Why on mileage check runs do they let the car run out of fuel on the track?

Question

They could put 7gallons in the tank instead of 5, a 2 gallon reserve controlled by a shut off valve, so they could get off the track, and not get plugged by an on coming race car.

Answer 1

It's all race strategy.

Answer 2

they are getting an exact calculation of how many laps they can run on

Answer 3

Its to find out exactly how far you can drive under race conditions.  Adding in a 2nd tank just for practice costs money, changes the way the car drives slightly, and of course its not going to pass inspection.

Everybody on the track knows when someones running a mileage check.  The team will tell all the other cars around them that they are expecting to run out of gas in the next few laps.  They also get out of the groove to avoid getting run over right before it runs out (fuel pressure will drop to 0 psi).