Would there be any benefit in mpg if I was to change my gears ratio from 3:73 to 3:08 on a 05 duramax Crew Cab with allison? Are there any problems this might cause?

I would think that it would be hard to find an 11" ring that size and I am not sure it would be a move you want to make if you do any pulling. I'd stick with the 3:73's

stick with the 3:73's..the change in power band will make the engine work harder.
I have swapped from 3:08's to 3:73's on my car and it was a night and day difference in torque managment and mileage didn't really suffer. You'll need the 3:73's for towing.

Although more expensive than a gear change, you would be better off to look into an auxillary overdrive unit. I'm not sure if Gear Vendors or US Gear has an application for the Allison yet, but they are working on it. This would allow you to keep your towing/hauling ability and give you an additional overdrive when running down the highway. These units usually have a 22% overdrive factor and when engaged you would realize a rear gear ratio of 2.90. Go here for more info

http://www.gearvendors.com/index.html or
http://www.usgear.com/dual_range.htm

Oh, BTW the change to 3.08s would be a :badidea:

JP