When your gas gauge does not work how can you tell how much gas is in the tank?

What I would do is fill the gas tank all the way up. Note the cost of the fuel and the amount of gallons it took to fill it. Clear your trip meter so that it is on zero. then run the vehicle for a day or two (depending on how far you drive each day) then refill the tank , note the mileage that you had travelled and divide the mileage that you had gone into the number of gallons it took to fill the vehicle the second time. You now have a estimate of the miles per gallon. Just keep track of the miles you have driven after that.