The fuel level gage won't tell you anything useful about mileage, though many people think those tic marks are magically precise. You record from full fill-up to the next fill-up, the volume added based on the certified fuel pump meter. Also record the distance traveled. The car's odometer is also not magically precise, unless you calibrate it against mileposts on the interstate. Instead, use those mileage markers on a long trip or your phone's GPS data for miles travelled. I regularly got 22 mpg in my 1969 Dart with 225 slant. That was in the days before 10% ethanol in gasoline, which lowers mileage perhaps 5%.