Automotive calculator

I need help in the automotive section with the gas savings calculator. I don't know how to get that formula on the topic. I see that you can add rows and columns, but I couldn't get it to work. What I was trying to do was show a calculator that would simply calculate your annual fuels savings if you trade in your old car and bought a new one. I traded in my 12 year old Maxima and bought a new Fiesta and my fuel savings is almost enough to pay for the car.