I no longer get the same mileage on a tank of gas. It was not a gradual change. From 1 day to the next I noticed I no longer get 320 miles per full tank. I only get 240 - 260 miles. what can I look at ? It is a 2002 Yukon 5.3L V8 with 184000 miles.

replace front fuel pump because gauge is made into fuel pump and not serviceable. Or poor boy way of getting by would be fill front tank with gas reset mileage drive normal for a day. refill again with gas note gallons used and mileage driven for average miles per gallon. front tank should be around 17 gallons multiply by mile per gallon you figured

completely unrelated
run the fault codes first then consider a problem heated oxygen sensor/s
consider a blocked cat converter/s
your stated average consumption is 20 mpg so you are either speeding or have a problem to start with
That car should be around the 400 miles mark per tank

One tank of gas is not enough to compare mileage. Or are you saying the computer's readout of mpg estimate is showing less? Try a tank or two of premium and see if that helps. Keep track of gas mileage. Driving techniques have a lot to do with gas mileage, as does long trips vs. short trips, highway cruising vs. city driving. For best mileage, do regular maintenance and keep the car in top running condition-good spark plugs and clean filters and clean oil ! Attend to minor problems before they escalate.
If you want to check actual miles per gallon, start with a full tank. Write the odometer reading down, and go. Everytime you put gas in, write down the amount. When you are close to 1,000 miles from the time you started, go fill the tank and include that amount of gas to your previous list. Now add up all the gas used and divide it into the miles to get miles per gallon. (976 miles/31.8 gallons = 30.69 mpg ) . Only one tank will skew your numbers, might be off cause the tank was filled more to the top than the previous fill-up. About a thousand miles of driving will give you a very accurate figure of miles per gallon for your car.

It just depends on your tank size. You probably get about 15 -18 miles per gallon. 13 gallon tank would be around 200 miles. 15 gallon tank around 232 miles.

change your spark plugs if they haven't been changed in awhile. check your tires for the propper psi there should be a sticker on the inside of your drivers door that will tell you recomended air pressures go by this not the recomendations on the tires. also where are you getting you gas does it contain ethanol sometimes this can burn faster.

could be either a bad thermostat or a plugged fuel filter try them first

Did this happen overnight or was it a gradual loss? Did you do any kind of repair before you lost the gas mialage? Is there a check engine light on or did it come on and go off? Did anything weird happen before the jeep got bad gas mileage? It's hard to diagnose a problem like this without actually touching the car.
Any other info might help me steer you in the right direction.

to increase gas mileage make sure you keep on top of tire air pressure you can also have your tires filled with nitrogen instead of air also a K&N air filter does wonders and is good for lifetime worth every penny. another smart move is to use a fuel system cleaner regularly Lucas makes great products for this try local parts store.

clean out the carb and fit an inline fuel filter between the pump and the carb .

I have the same problem as talked about above. 2002 Hyundai accent gsi. HORRIBLE gas mileage. I only get about 320 km per full tank. My driving habits are good. i took it to the garage and they put it on the computer and no error codes showed up. Question: Can a MAP sensor be faulty but NOT show up on the computer diagnostic? I had my O2 sensor checked out and my exhaust, and changed the flex pipe. Car runs pretty good just horrible gas mileage and kinda crappy power. Changed fuel and air filter. Cleaned the fuel system entirely ($80 at garage not $4 bottle).

Anyone ever use a resistor in the MAP sensor?