My rig is a 2004 4x4 Supercab 7.3 (in Australia), I have just installed a 4'' lift and changed from 33'' tires to 35'', same brand, BFG Muds. Prior to the lift my mileage was around 17L/100km (around 13.5mpg) and now with the bigger tires I am using around 24L/100km (9.5mpg). The running is mainly city work with some highway hauls. I am normally carrying around 0.7 to 1.2tonne (1500-2650lb). The truck has 57000km (35000miles), and is due for service, although I have replaced the air filter thinking that may be restricting things with no noticeable change.
Hope I hoved given enough info for someone to help.
Cheers Mick9

 

Sorry forgot to mention that I have allowed for the increase in rolling diameter on the larger tires into my fuel figures.

 

No wizard here on the PSD's but I bet you have loaded up the PSD with the addition of the bigger tires. Which with the stop and go of city driving will really show up with this style of driving.

Now with that many miles on her you certainly need to replace the fuel filter. I like to change that every 25,000 miles or sooner. I would be for checking the small screen for debris from the fuel system like pieces of O-rings as well. This small screen is behind the FPR. I know the first time I checked mine it had quite a bite of debris on it which alarmed me, but I have not seen much of anything since and I have a 10 year old truck now that has had this screen checked every 25,000 miles... I have 132,000 now...

For as the new larger tire sizes and getting better fuel mileage, I see guys all the time around here get what I call pretty low mileage with the 3:73 gears and big tires. Even the new 6.0's seem to get poor mileage with the 17" and some of the limited addition 18" tire sizes being put on them by Ford...Let alone what some of us change them out too.

Most of us like the taller tires alright and you might look at doing some checking into a few mods on the air intake and exhaust system to see if this might help. I fear you may need to look into 4:10 gears for really improved fuel mileage though. A good chip program will help some but only if you have the first things added first and then, most important is drive her as you do now. Which after a host of mods here on the 7.3 PSD, I think most of us here agree is very hard to do LOL...Driving the PSD and in a normal way is very hard to do with what power improvements can be had from the PSD...