Dually tire questions
 

I am insterestred in lifting my 2005 dodge dually. 37 is about as big as I wanna go. is it possible to get taller tires without getting wider tires. The truck is a puller so MPG is a factor. The goal is to lower engine rpms at hwy speed.

Yes you can get taller tires, 295, 305, 315's but for the cost of a e rated 315 or 37 is quite a killer on the pocket book and you probably will not see a huge increase in fuel milage maybe .5-1mpg. Bigger tires take more horsepower to turn.

is know it will not save me money in the long run but it will look awsome. Also do bigger wheels make a difference in fuel economy say 17s vs 20s

As long as you calculate the correct mileage..

Buy a splitter to lower the rpm, get 19.5 wheel and tires and increase weight capacity and tire life. I don't understand how bigger tires and wheels will help with mileage, while towing. You would get worse by changing your gear ratio like that, unless you are running empty and cut weight with lighter tires and wheel. Sounds like two different goals.

Can't see how going that big will do anything but hurt millage and stability. Larger wheel will help with stability but millage will hurt. Do you plan on regearing? What are you pulling? Weight, etc. If you think going to a 37" rubber and have 20 mpg is going to happen, think again. It's mainly gearing, then all the air being pushed... and mainly going under the truck. These engines have a sweet spot for RPM's & mileage.
There is a reason why hyper millage guys run extra air dams up front. Another reason why 2wd gets better millage. My guess is you want millage to not cost so much running down the road. Are you getting the new rims, lift kit, and tires (and maybe gears) for free? Remember changing the size of the tires changes the gearing... all the gears, first gear, second gear, etc
Think of what you want. Looks of a lifted truck or millage/power.
If money is no object, regear, change the rubber, then add a overdrive unit.