MPH VS RPM

[Sierra4x4]

On My 79 GMC Sierra 4x4 I run 55-60 MPH @ 1900 RPM's. Im running tiny 32 inch tires w/ the 4 speed manual tranny. I drive the highway everyday. And Im not even doing 55-60 MPH more like 47-52 MPH. Sometimes I wish i had a 4th gear in there.
Anyone else with the same setup? Just wonderin what everyone else runs.
I plan on puttin 37-40 inch tires on there in a year or two (when i can upgrade the axles) I read another post that about a similar topic that said put on bigger tires and your RPM's will go down, its true but you aint goin as fast. I often wonder what would happen if i threw on 235's.  

[Eric 85K10]

Is yours a K10?
I'm running the same tranny in my 85 k10, I'm not sure what the rear gears are since my speedometer is 5mph slow, if I'm doing 60, my guage reads 65. Stock its suppose to have 3.08 gears. My clutch on my fan was siezed up so my fan was spinning with my engine and it roared at 60mph, but now with that fixed I'd guess I'm getting around 2000 rpms on 31in tires. I'm afriad to upgrade my 31s to 33s with out switching out my 305 first.

I do know if you put bigger tires like your talking about you will want lower gears in your axles to handle them.