What is the actual fuel tank size? I read it was 15.4 but on my cars' computer it says I have used 10.1 Gallons (manually reset at last fill up). When I look at my onstar app it says I have 3 gallons left and a 24% tank level. Thats only a total of 13.1. Is the entire tank 15.4 leaving 2.3 gallons for reserve?