Why can i only fit 13 gallons in my 19 gallon gas tank on my 2002 jeep wrangler sport 4.0

You'll need to know how big the fuel tank is on your truck. They used different sized tanks depending on the box or body fitted. I think the standard body box was a 26 gallon tank and the long body box was a 34 gallon. This will help you work out your actual miles per gallon.
That model's factory figures have it at between 12 to 13 mpg average. How and where you drive your truck can also affect you fuel consumption quite a lot, City driving, Hard accelerating, Lots of steep hills where you drive and Driving Off Road all can ruin you MPG figures. Using the your numbers that does seem a little lower than it should be. When was it last given a full service? Have you changed fuel supplier, (different gas station). All of these affect MPG Do you have any warning lights on the Dashboard? Use the comment button below with more information and I'll try to help you more.

Using your numbers here's the math.
26 gallon tank - half tank = 13 gallons. 34 gallon tank - half tank = 17 gallons. You got 104 miles.
Sum used is 'Miles achieved divided by the number of gallons used'.
(26 tank) 104 divided by 13 = 8 Mpg.
(34 tank) 104 divided by 17 = 6 Mpg.

Sometimes the short trips are not representative of the actual MPG. When a low tank has a few gallons it can skewed the Distance To Empty figures. The first 8 Miles can take a Gallon, until all the sensors warm up. You had probably 4 Gallons(19 MPGx 4=76 DTE). Now you have 2.5 Gallons left (19 MPGx 2.5=47DTE) to 2.8 Gallons (19 MPGx 2.8=55DTE). So for a low tank you can see that the DTE falls in the middle of the approximate DTE you stated at 50.

Long-term, you should probably clear all Codes with a free scan at Autozone, Advance, or Oreilys autoparts. Sometimes a Code is picked as the Dominant code and there can be another that has not malfunctioned enough to set and stay.

Also, the old malfunction could have reduced the efficiency of the catalytic converter either temporarily or permanently. It will take quite a few miles for the build-up caused by the rich mix to burn off.

The wires going to the new O2 sensor are also important. The O2 sensor has a heater in it to quicken the change to a lean fuel mix. It needs to work and attention paid so that the O2 sensor is not blamed for a bad wire.

watch your fuel gauge they don't always read true they work on ohms if bottom of rod is worn the float isn't making good contact

add more gas to the tank; until you have at least 5 gallons in it.

Fuel Info Fuel Tank Capacity: 23 Gallons. EPA Mileage Estimates: (City/Highway/Combined) Automatic: 13 mpg / 19 mpg / 15 mpg Range in Miles: Automatic: 299 mi. / 437 mi. / 345 mi.

Your tank is 19 gallons and you get about 18 miles combined miles per gallon. Thats city and highway. Highway is around 21 and city is about 16.

Most gauges are set up this way to give you a bit of "margin" between "e" and stuck on the road.Sometimes this is set a bit too high but adjusting it is not worth the $. Additionally, there is generally about a gallon of fuel remaining in tank that the pump cannot reach. This protects you against most heavy debris (rust etc) and water that acumulates there (even in the most well maintained vehicles) which would otherwise easily enter the fuel system. As long as you are not running out at 1/4 tank or anything, you are OK.

You will actually have about a Gallon or so of Fuel left even when you get to the E Mark. Even if you run out of Gas there will still be some left, But the pump can't pull it up from that far down.

sounds like your fuil pump float is stuck

Have you taken it to your mechanic to run the diagnostics... I had the same issue and it turned out to be a vapor lock issue that required a gas tank gasket replaced.