The computer counts the number of times the fuel injectors fire to estimate the fuel consumption. It's pretty accurate but not perfectly accurate. I agree on the 17+ gallon tank. You have to consider how much fuel is in the lines and how much you have 'over-filled'.
I have several times driven well past "0 miles to empty" with the most being about 15 miles past that point, yet I have never been able to put in more than 15.9 gallons of fuel.
2005 Mustang GT with 300 ci 04 Cobra iron block, Kenne Bell 2.6l twin screw, P&P S3 Heads, SS valves, Comp S3 cams, Currie 9+ 9" rear end and DS, Hurst shifter, BMR K-member, A-arms, LCAs & UCA, JBA long tubes & mid-pipe, Accel COPs, Dual GT500 fuel pumps, much more...
670 RWHP, 640Tq @ 16 psi & 6400 RPM