When I bought my 2000 F-250 some 10 yrs ago I was getting about 18 mph. I'm now down to 10 with 125,000 miles. Nothing has been done to the truck except the oil changes at normal intervals. I have began using a high mileage oil.
Any ideas of what could be done to bring a few of them mph's back?

 

Gas or Diesel? Auto or manual? Cab type? 4x4? Tires? Etc? Any codes or lights? Give us as many details as possible so we at least have a place to start. So many possible causes of lower fuel economy, some beyond our control such as ethanol and low sulphur diesel that can also affect fuel economy.

 

5.4 Liter gas. Club Cab. 8ply highway tires (pull 28 foot trailer)
Automatic

 

2 wheel drive
No warning indicators

 

Club cab? I sense a Mopar in the midst.

 

Nope not mopar. It is the cab style that has the less than full size rear doors if that is a better description. But definately not mopar

 

Have you done the other regular maint? fuel filters, trans maint, spark plugs, clean the MAF sensor, air filters? what about exaust work? Any cat's plugged and changed. If you just did regular oil changes that is ok but other things need regular maint as well, good luck.

 

I've been leaning toward the cat converter and exhaust leak for sometime. Other maint been done but not sure what MAF sensor is.

 

Mass air flow. It will be in the intake tube after the air filter. It needs to measure how much air is comming in so it can mix fuel properly
http://www.youtube.com/watch?v=gd640ImE9Ec not the same vehicle but the info is good

 

Mass Air Flow sensor.

I would not throw parts at a potential "problem" hoping for a hit.

 

Don't forget about the PCV and rear diff fluids.

 

Spark plugs would be a good place to start if they have not been changed. It may not be THE problem but it will definetly help if they have never been changed. A new air filter would also be a good idea if it has not been changed out either. I would address those first and see if it helps