If it hasn't happened to you yet, it eventually will. One day, you'll start up the car, and even with plenty of gas in the tank, the fuel gauge simply won't move. The good news is that it's probably not the gauge itself so that typically eliminates messing with the dash cluster. But the bad news is, the fuel gauge sending unit in the gas tank is probably shot. Now you may wish to have a mechanic do this for you. The tank must be lowered in order to get the sending unit out; lowering the tank is usually easy, getting the old unit out isn't. It's usually rusted in the tank pretty good, so getting the snap ring to cooperate with you is a chore. Otherwise, it comes out just like a fuel pump.
In the meantime, you can use your trip odometer to calculate your gas mileage. For V6 and V8 cars, you should have about 1/4 tank at 225-275 miles. For the turbo-4 XR7, you have a smaller gas tank (18 gallons vs. 22 gallonsfor the V6/V8) but you also get better gas mileage, so you can go to about 300 miles with 1/4 tank left. It's not perfect but this will generally work well until you get the sending unit changed.
By all means, if you have any qualms about tackling this job yourself, DON'T do it. Take it to a shop and pay the money; it's worth every cent for you not having to mess with it.
If you are going to change the sending unit yourself, though, here's some info you'll need to know.
– When getting a new sending unit, be sure to specify whether you have an electronic or analog gas gauge. There is definitely a difference between the two, and if you end up getting the wrong one, your gauge will work the opposite of normal.
– The resistance for fuel level sending units for our cars is as follows:
Analog: 60-86 ohms empty, 8-12 ohms full
Electronic: 10 ohms empty, 160 ohms full
– It is possible to have your stock sending unit rebuilt. Check your local shops to see if they offer this service; if not, a quick search engine query should help you out. You may need to send it away, and it will take some time to get the unit back, so plan accordingly.