My 2004 G35 Coupe
This car has a 20 gallon tank. Since it gets 25 mpg on the freeway, one should be able to get 500 miles on a tank of gas. Indeed, after driving with a tank of gas on the freeway, the Navigation System computer will say you are getting 25 MPG, and when you fill up after a freeway drive, it will say you have around 500 miles to empty, say, 495 miles. Starting with this 495 figure on a freeway drive, after you have driven 250 miles on you trip odometer, the computer will say you have 245 miles to empty, as it should. After 375 odometer miles, it will say 120 miles to empty. Then it gets strange. Drive another 20 miles, and instead of 100 miles to empty, it says 80 miles to empty. Drive another 20 miles and it will say 45 miles to empty. After another 20 miles, and it says 15 miles to empty, and it is screaming at you to find a gas station!
I once tried to test it on a straight freeway drive from Tulsa, OK to Houston, TX (502 miles). About 60 miles north of Houston, it said I had NO MILES to empty, even though by my calculations I had another 60 miles in the tank. I chickened out and stopped for gas, thinking it would take close to 20 gallons. I filled it "to the brim" and it only took 17.6 gallons, and I had driven 440 miles. 440 divided by 17.6 = 25 MPG, so I really had another 60 miles left (2.4 gallons X 25MPG = 60 miles). Either the gas gauge is wrong or it doesn't really have a 20 gallon fuel capacity. I tend to think it is the gas gauge, as I was recently running low on gas and got the "0" miles to empty warning, and it only took 16.2 gallons!!!
This happens in my car FX35 as well. Your fuel capacity is correct but the gas gauge is not very accurate but I won't blame the car companies. Fuel injection pump is immersed in gas tank. Running low on gas for a long time frequently my damage the pump life so it is advised to fill up as soon as you see the gas light. My car shows "***" after it reaches certain miles in a tank. I was able to drive another 20 miles and yet it didn't take gas up to its full capacity when I filled.
I bought a 2004 G35 sedan new, and just put it through its 150,000-mile service. I don't think I've ever gotten 18 gallons in the tank, even when the warning light had been on for an hour, and the entire gauge pointer needle was below the "E" mark. They just don't want you to run out of gas.