my friend has a 98' dodge ram with a 5.2L 318 engine.  he says he gets like 100 miles off the first quarter tank, and then like 120 miles to the next three-quarters tank.  basically his gas mileage goes from 13.5 miles to the gallon to 8 miles to the gallon.  i suggested his fuel injectors could be bad.  but i dont know.  

His fuel pump sounds like it might be clogged.

Have him hand calculate the miles, bet hes getting the same milage, just the guage or tank layout is making it appear that way. I know mine drops like a rock after 1/2 tank, just cause of the sending unit setup  

The rate of fuel consumption isn't changing. He needs to calculate mpg on the whole thank. There is no guarantee than the half mark on the guage indicates the true volumetric half level of the tank. The float in the tank only indicates the vetical level of half. It is very common for the lower portion of the tank to have physically smaller dimensions, and therefore contain less fuel than the upper portion.

My truck goes 200km before it comes off the full mark, about 300km from full to "1/2" and the bottom "1/2" only takes me about 100km. It is just  a poorly designed float armature. 

no its actually not the guages, the truck actually runs really rich.  its obvious.  could this sound like an O2 sensor problem?  

yea i know that guages can sometimes do that, but his truck actually runs rich the last 3/4 tank.  could this be an O2 sensor problem?  

The O2 sensor would have no idea how much fuel is in the tank. The ECU doesn't monitor or process fuel level against other parameters.

Replace the entire sending unit and put in a new fuel filter.

start easy- First check the air filter, if it is dirty it will cause the engine to pull more fuel. next the injectors should read 12 to 15 ohms.  and yes it could be an o2 sensor.  these trucks have two of them.  if they are sending the wrong info to the computer it may think it needs more gas.  a plugged fuel filter will usually cause the engine to sputter.  Other things to look at.  thottle positioning sensor, mass air flow sensor and a plugged exhaust.  i have also seen that if your tranny needs a good cleaning it will cause mileage problems as well by lugging down the engine.  

thanks for the help man  

HAVE HIM FILL IT UP AND RUN IT WITH THE GAS CAP OFF.  IT COULD BE A BAD VENT INSIDE THE CAP CAUSING SUCTION WHEN THE FUEL LEVEL DROPS.  THIS WILL MAKE THE FUEL PUMP WORK HARDER, DROP THE FUEL PRESURE, AND CAUSE A LEAN CONDITION.  THE ECM THEN RICHENS THE MIXTURE TO COMPENSATE.  HE MIGHT ALSO GET A SUCTION SOUND WHEN HE REMOVES THE GAS CAP.  GIVE IT A TRY.  

Its not an O2 sensor, i got those replaced and its the same. its my truck by the way...

Another idea;

Gas tank is not venting properly, forming suction inside the tank, forcing the the fuel pump to work harder, computer pulls timing to accomodate less fuel or lower fuel pressure, rich condition and poor mpg ensue. To correct this; the Carbon canister may not be funtioning any more, try disconnecting the fuel tank vent line from the carbon canister, and putting a small line filter on it and see the results over a tank of fuel. If this corrects it, to stay smog legal (or pass any sort of emissions screening) Replace the carbon cansiter and any associated carbon cansiter valves.  

Quyonmob.

wow this has been very helpful. I also have a 98 Ram with simular problems you mentioned tranny cleaning what is that all about I have have changed the fluid and the filter is there something else that goes along with that?

thanks for info on this It has given me more this to try to get my truck running right.

as you may have heard - the dodges of the time era are known for tranny problems.  my dad has a 98 dodge 1/2 ton with the 318 and he has had some of these same problems.  the last time i did a tranny flush on it he was having sputtering problems before we started the job.  after flushing, took down the pan and filter.  i decided to look at the vavle bodies which turned out to be pretty clogged up.  took them apart and cleaned them, put everything together.  filled with synthetic tranny fluid ( the only way to go) and the truck run great after that.  I don't use anything but synthetic fuilds in all my trucks and cars for a very good reason.  

Thanks for the advice. how hard is it to take out the valve body? I am going to assume when you say flush the tranny you mean putting in an addative then changing the fluid and filter.... I am I correct in assuming this?