My gas gauge is going down too fast, but the display is still reporting reasonable mileage.

I had an oil change where the maintenance signal had to be reset recently, but otherwise nothing has changed.

My first 80 miles ~42mpg are reported, but the gauge is only around 5/8.

If somebody stole gas out of my car or there was a leak in the gas line, would that affect the display?

 

Nobody bothered to answer this in 4 years?

I don't know how exactly the car calculates the MPG on the display. My guess would be that it should report lower mpg if you are losing gas.

I'd recommend you calculate the gas MPGs yourself to confirm that the display is correct. Fill your tank. Before you pull away from the pump, reset your trip odometer. The next time you get gas, fill the tank again. Take note of the amount of gallons it says on the pump (or get the receipt). That is the amount of gallons you used for the distance that accumulated on your trip odometer. Divide the trip odometer miles by the number of gallons. So, if your trip odometer says 300 miles, and you pumped 8.000 gallons, that would mean you are getting 37.5 MPG (300 ÷ 8 = 37.5).

I calculate mine every time I fill up, just to make sure there's nothing wrong with the car.