Okay, I've looked around and I think I know the answer. However, I would love it if somebody could give me an easy fix. I have 133 miles driven, and the fuel light comes on. I put 10 gallons to fill it up, and the fuel gauge doesn't quite get to full. I know it's a 20 gallon tank, so obviously the fuel gauge is off. That probably means the resistor inside the fuel tank is reading wrong. But is there any way to fix it, or any other options without having to drop the tank? You guys have been great, and thank you for your reply!
This is for a 1996 Jeep Cherokee Sport.

 

You can check the resistance readings for the sender.

Fuel Gauge Calibration
Pointer Position Resistance
Empty Graduation (+0° / -4°) 105 ohms
1/2 Graduation (±3.5°) 33 ohms
Full Graduation (+6° / -0°) 5 ohms

LOW FUEL WARNING LAMP
A Light-Emitting Diode (LED) on the face of the fuel gauge unit will light when the fuel level falls below approximately 4 gallons. A low fuel warning module attached to the rear of the fuel gauge controls when the LED will light. When the module senses 66.5 ohms or more resistance from the fuel level sending unit for 10 continuous seconds, the LED will light. When the module senses 63.5 ohms or less resistance from the fuel level sending unit for 20 continuous seconds, the LED is turned off.

So if the low fuel was on, you had 4 gallons or less. Add 10 gallons and it's only 3/4 full (or less). Of course, you can't be sure what level it really in the tank unless you drain it.

Also bear in mind that the '96 fuel quantity sender has 2 sets of resistor coils. One for the gauge and one for OBD2 support. So the unit is unique amongst XJs

 

I do believe you can remove the sending unit with the tank still in the jeep...

Whats the overall history of the jeep? How long have you had it, have you done any work recently? Is the tank dented?

If the float level arm has been bent - it would read incorrectly. If the tank is dented and hidden by a plastic or steel guard/skid plate it could be inaccurate. If someoen has doen the extended vent tube mod to the inside of the tank you could actually be holding more fuel and therefore have inaccurate mileage. Soo many possibilities!

 

Thank you all for the great information! The Jeep is in good shape, no damage to the tank. Although just the other day a young lady just crushed my rear hatch. Frame and bumper are fine. So I'm going to be working on trying to find a new/used hatch! Yay.