I always wondered about this myself.  My ST is a 8-2020 and I'm supposed to have a 21.8 gal fuel tank.  Once I was super busy where I did not have time to stop for gas, so when I was able to, my dash read out said I had about 8 miles to empty.  I filled tank but it only took 17.9 gal to fill.  Maybe there is a substantial reserve but I never was able to figure that one out.