08 f350 srw 13-15 city, havent got it on the highway much yet... 3.73 gears too...

Me too! My 04.5 Duramax only gets 10 towing 10k @ 75mph & 15 on the highway, empty.
I have no lift & larger than stock tires.
I can't believe Sean H is getting that kind of mileage with those big tires!

Im getting 12 around town and thats with 3.73 gears. My 07 F-350 had 4.10's...13.6 mpg and I wish I would have gone that same way. We will see, only have 1400 miles, needs to get it broken in. Everything else about the truck is awesome and I prefer it also over my 07 GMC Duramax for towing the boat. Not as much power as the all mighty Duramax, but is much more stable and stays planted on the road when pulling the Nortech over my GM's. Jeff

How do the Fords ride empty? Compared to a GM.

the tires lower the cruise rpm a few hundred revs.. probably helps more than hurts when it comes to mpg... acceleration is hurt a little... got 13.9 on the hand calculator the last fill up, about half city and half highway. my old 05 got 16 and 19 with exhaust, programmer and intake.. havent done anything to this one yet.. the new ones ride and turn better than the old ones and the old ones were better than a chevy....

The GMC rides better than the Ford..thats 07 to 07 and both dually's....the 08 F350 dually rides better than both the 07 Ford and GM...BUT dont have a 08 GMC to make a fair comparison..

I bought an f450 4.88 6.4 diesel because it was the only option to legally tow my boat without going back to a heavy duty (or medium) commited tow vehicle. I have been very satisfied. It is as quiet and smooth as any car, has an incredible turning radius, and can be used as a daily driver even in urban areas. It tows my boat under better controol than the International I used last year.

Wait till warm weather to check your mileage. You Northern guys anyway. The winter blended fuel cost me 2mpg.

To answer your original question they get roughly the same mpg ( assuming the gear is the same). They have the same motor & trans. I had a 350 4x4 dually last year & now I have a 450 4x4 but went to the 4.88 gear. I lost a couple mpg but they do get better the more they break in new I was at 7-8 mpg now at 2500 miles I'm at 10 mpg in the city.

Ive noticed that also. 9-11 when new, now I have 1500 miles and getting a solid 12. I have been told mileage goes up drastically on job 3 trucks once you hit around 15-20k, per a few of the guys that haul weekly. Also Ford is looking at a flash to help in this dept as well.