I have a 96 f150 4.9ltre. truck has 251k and lately i have noticed that on the rear gas tank, If I fill the tank completely full, usualy only takes about 14-15 gallons (figure there is about 1gallon un-useable) I can only drive for 200 miles. but If I only put in 5 gallons can get 100 miles. Why is that? can someone help me on this issues. Is there a fuel tank pressure problem?

 

Sometimes one tank will pump into the other. Ford makes a check valve kit for that problem.

 

remember u own a truck not a honda.. we dont buy them for fuel economy

 

THANKS FASTER, BUT WHEN YOUR PAYING IN THE UPWARDS OF 2.50 A GALLON AND ONE WEEK YOUR GETTING 18MPG AND THE NEXT YOUR GETTING 6MPG EVERY LITTLE BIT HELPS. i CHECKED THE FRONT TANK. ITS STILL BONE DRY. THE PUMP WENT OUT A YEAR AGO AND I DONT WANT TO WASTE THE MONEY BUYING ANOTHER JUST TO HAVE IT GO OUT AGAIN. I WANTED TO SEE IF ANYONE KNOWS WHEATHER THERE IS A TANK PRESURE VALVE OR SOMETHING. I KNOW ON MY WIFES GM HER TANK PRESURIZES FOR THE EVAP SYSTEM AND HER CAP CAN CAUSE MIL LIGHT TO COME ON OR BAD ECONOMY. IS THERE SOMETHNG LIKE THAT IN THESE TRUCKS? THE GUY AT THE AUTOZONE TRIED TO TELL ME THAT 5 GALLONS WEIGHTS LESS THAN 16 GALLONS AND THAT WOULD BE MY PROBLEM. I TOLD HIM HE WAS FULL OF IT. THAT THERE WAS NO WAY 10 GALLONS WORTH OF FUEL WEIGHT WOULD CAUSE A MPG DROP OF 7MPG. WHATS EVERYONE ELSES TAKE ON THIS?

 

well buck up.. i drive 300 miles a day to and from work.. and im gettin 13.7 mpg..

 

Are you sure that the tank is empty, and that the sending unit or guage are woking properly? Unless you have a leak in the tank then there is no way that filling the tank will cause your mileage to drop.

 

yes I stuck a siphon hose in there and its not picking anything up and it isn't even wet. I even took off the filler hose and lowered the tank and heard no sloshing so I know its bone dry.
I know that it sounds stupid and I have never heard anything like that before either but I drive 40 miles a day with limited intown driving. like 2 stop lights. and I can do 2.5 trips to and from work. which is exactly 100 miles. (found out when I ran out) but fill the tank to click and I get 200-215 miles till run out. (I carry a jug full just for that purpose.) If there is 4 quarters to the gauge and 5 gallons fills up to about 1/4 or less then shouldn't full be 4 quarters close to 16 gallons? since 16 is divisible by 4 . so 1/4 gets 100 miles then 4 quarters should get 400 miles. I know that its not a perfect world and that would be too perfect but even 300 would be good. five gallons means my truck is getting like 18-20 mpg but full tank is like 13-15 mpg. doesn't make sense. Can some else try this theory and see what they come up with.