Am I doing the mile per tank math wrong?

I have a Ford Taurus 2002 and it's in bad shape, I know that. The internet says it should have 18 gallon tank and 20 mpg city. My math would be 18gallons X 20miles=360 miles for the tank before needing a refill oif all good. But I figured I've driven about 78 miles the last 3 days and have had to put in about 42 gallons of fuel in it. Am I doing the math wrong or do I really only get 1.85 miles per gallon from the car (78miles/42gallons=1.85mpg)

If you drove 78 miles and had to purchase 42 gallons then you have a serious fuel leak or someone is stealing your fuel. No way would the engine burn this much fuel in 78 miles.

If your gallons and miles are correct, you've got a horrible guzzler on your hands. You must have fuel coming out of your tail pipe.

Get gas and top off the tank at a LEADING brand station. Set the trip meter to zero. Gas up again and top off when the gauge is 1/4 full. Note how much you put in the tank. Check the trip meter. Trip meter reading divided by how much fuel you put in is the mileage.

Buy a locking gas cap for your car. Your fuel mileage will increase dramatically.

Even a formula 1 car is not that thirsty. As others say you must be having fuel stolen… Leaking that much you would smell it

That car is in very bad shape. Fill it up once again. Drive 10 miles and fill the tank. See how much gas fills to tell what you used in 10 miles. Calculate from that. Maybe the fuel gauge is no good, if that's how you've been calculating the amount of fuel you bought.

The only way that could happen is if you have a leak.