Right now I'm getting 44 MPG combo, city/highway (2006 RK). I try and run it 35-40 MPH in 5th on secondary roads and 70 MPH on the highway.
If I would slow it down on the highway would my mileage go up?
If I run 4th on secondary roads would my mileage change?

 

Generally speaking. as your RPMs go up your mpg goes down.

 

My gas useage goes up if I run 5th at those low speeds...seems I have to dump a lot of fuel to get it up even small hills.

I almost never run lower then 2500 rpms regardless of the gear and will average 37 to 40 MPG city driving...but that's on an '08 UC.