puting smaller tires will help your fuel milage. if you want to put some money in it and get a little more power and better fuel milage out of it a chip and maby 90 hp injectors will help along with exheust and intake. i got about 3.5 more mpg with my dodge just by buting a chip in it the trick with that is you just have to keep you foot out of the throttle hard to do some times

yes your gas mileage would be horrible with that kind of setup
those trucks might get about 18 mpg on the highway and about 13-15 mpg city. the mudders will probably make you get about 3 to 4 mpg less than that plus the lift will throw off steering and the spedometer. this truck probably isnt a great vehicle for daily driving.

I would bet that you would be looking at about 10-12 mpg’s depending on how hard you drive.