Yes, the gauge is non-linear. The tank shape is not a perfect cylinder (or cube). And, the sensors age over time, so they don't read perfectly. You absolutely cannot measure from "half full" to "full". In fact, you should ignore the fuel tank reading. Fill the tank. Run it to down to the redline. Fill to completely full. Note the mileage. Run another full tank of gas. Divide the number of miles driven by the number of gallons filled. Do this every time you fill your tank. You will find mileage varies significantly based on driving habits.When you say "The gas gauge is incredibly non-linear" Do you mean that it appears that from half full to full is actually less gallons than from half full to empty even though it should be equal? I've been measuring my mileage from full to half. I can't find if there is a on dash mpg display.