Your going to think I'm nuts when I mention this..
I've got a 07 F-450 with a service body.. It's loaded heavy, I drive it almost daily back and forth to work, when I get tired of driving it I switch to something else ( either the '00 F-250 or the '05 F-350 ).. Even then, I'm typically in that truck working 12-14 hrs a day. Often times I use it to pull one of the cars to the track because it has 99% of my tools in it.
It's bone stock other than the Edge W/Attitude. Runs 245/55 19.5's ( I might be off a little on the size ). The wheels are crazy heavy, steel 10 lug wheels. Has 4.88's gears..
If you keep the truck a tad under 65 MPH, I've seen this truck get 20 MPG many times.. But if you go over 65, or your driving in the city, idling and what not, typically I see 12 MPG, it's even been in the single digits a few times.
When I bought this truck, I figured it would get horrible fuel mileage, and it would only be used when I have to work at railyards and such. A 1 ton wouldnt have been truck "enough" for what I needed to do, so I was screwed basicly. After a while, I started driving it daily because it did better than my "daily drivers".
It sounds nuts, even more so when ya look at the size of this thing...
My 05 F-350 4 door SRW is lucky to get 15, it's got exhaust, Edge Platinum and EGR delete.. I was towing today, hauling round bales. 14K LBS load, plus trailer, plus truck, it got 10 MPG driving 60 MPH. I hauled 50 bales, 10 at a time, 50 miles round trip.. It's averaged 14-15 it's entire life.
My '00 F-250, I used to get 18 out of it ( I've seen 20 a few times, it got 22 one time ), before I tore it down to do a BUNCH of maintaince ( including injectors and such ), I only got 12 on a good day. If I can get it back up to 14-15 I'll be happy. The odd thing, I could hook up to my 35ft enclosed G/N race car trailer, with car and supplies for a weekend, on a 800 mile round trip, it got 11 both ways. Yet only got 12 empty before I tore it down. It's got a 4 inch lift, running 35's.
A pic of the F-250 and the trailer I was talking about.. Truck getting 12 empty, but 11 with this trailer
Obviously has ( or should I say had? I hope ) problems..
So what am I saying here? Mileage from one truck to another is a total crap shoot. A F-450 should NOT get that kind of mileage weighting what it weights ( best guess is 14K ), but it does.. A slightly older truck, same engine, weighting much less, gets worse mileage ( running close to the same RPM at the highway speeds I run them ).
Swapping over to SRW, I really dont see you gaining a single thing from it.. In all honesty, by the time you swap that out, have the body work done so it looks right, your better off finding a different truck. Your truck is worth more now ( to the right person ) than it would be if you modify it like that..
I'd look into a different solution, but even then, that might be what your stuck with. Toss all kinds of parts and money at it, it could get worse, you just never know..