Getting Bad MPG

It seems to me that my car is getting like half of what it is supposed to get. I have a 1995 Nissan Altima and like 4 days it almost hits empty and im not sure if it's just a sensor misreading or not. I was just about at empty again and refilled and forgot to see how many gallons i put into it. If the sensor is a problem are they expensive to replace? I know i need to fill it up and record milage till its empty. I know this could probably be a range of problems but any advice is nice. Right now it's in the repair shop for a transmission problem.

until you know what mileage you're getting you won't really know if it's off or not. also driving habbits have alot to do with gas mileage, i usually refill my tank every 6 days but there have been weeks where a tank would last me 8 or 9 days because of weather, traffic and driving habbit. also other times i've had to fill up in 4 days because i was consistently booting it to school.

once you know what mpg you're getting we can help you figure out what's wrong and if there is something wrong.