I'm looking at a 2005 f250 6.0 xlt with 55000 miles on it and I want to no how the moters are and how reliable they are and what the mpg would be like

 

5500 MILES ??? awsome.. There are a couple things to do right off the bat and you won't have to ever worry about it unless you add too much power to it.
Replace the STC fitting on the HPOP, replace the Dummy plugs and standpipes with the upgraded ones/ total cost here is around $140 if you do your own work, then do a EGR delete and add a coolant filter and it should last you 300k more miles. and if you really wanted to do it right then send the FICM out for repair/preventive repair/ and you are good to go.
Or you can wait till the STC fitting fails at about 140k, but ADD the coolant filter ASAP to save you from having issues caused by a clogged oil cooler. clogged oil coolers can cause multiple issues but if you get the coolant filter on it right away you will save alot of grief later.

 

They are great if you treat it right. With the right upgrades I think you can get around 20 mpg most will say other but that my opinion.

 

mines got 70k right now and when I first got it I got 20.4 mpg on the interstate with only an air intake and cat back exhaust

 

Almost everybody says the dash is way off with the mpg calculations which I don't see how. It will tell me for example 580 miles to E after I fill up and I will get roughly that many miles out of a tank.

 

Curry,

How long have you had the Atlas 40 FICM?
I would think this is a significant reason for your mileage..another would be your EGR delete.

I line in Colorado and have done a number of upgrades, but, because Colorado requires a smog check I've stayed away from EGR delete and FICM tuning...the best I've seen for mileage (hand calculated based on full tank each time) is 17.2.

 

Im not sure 6 months maybe? That really sucks you have smog test.