Wishing I had overdrive...

[shahram]

I own an '03 4WD Xterra All-Manual, stripped, bone stock.

In 5th gear...At 65mph, I'm at 3000rpm. At 70mph, I'm at 3,250rpm. And at 75mph, I'm up around 3,500rpm.
Since most clear highway speeds in SoCal are up around 75mph, I am running constantly around 3,500rpm. From listening to the engine at speeds over 55mph, it sounds as if it's well past it's peak efficiency (in other words, it's revving too high). If that's the case, it's causing more wear-and-tear, not to mention lower fuel efficiency.
My question is: do you X owners with auto-trannies have an overdrive option that brings your rpms down while cruising? Is this something that manual tranny owners have to live without?
And finally, why did Nissan make a V6 with peak 5th gear efficiency (mid-2,000's) well under normal freeway speed limits?
What is the advantage to this type of gearing ratio? Was the X just not geared to drive efficiently on the open highway?

Also, how will the gearing ratio be affected when I replace my stock "Grabbers" ("Crappers") with decent 31" BFG AT TA/KO's?

Oh, and since I did too many drugs in high school, please break any answers down into (SIMPLE) layman's terms. Thanks!

[OffroadX]

You do have an overdrive, 5th gear is .811:1 and is by definition an overdrive since the output RPM is higher than the input RPM.
The 4-speed automatic has a .694:1 ratio, a much taller overdrive. Not sure exactly why, but I'm sure it's only possible due to the torque multiplication of an automatic. A manual probably couldn't effectively push a gear that tall.
My 2000 automatic spins just over 2500 RPM at 65 MPH.

Gearing will be negligibly affected by going to 31s, you have the equivalent of 30" tires already, and the same driveline pushes 265/70-16 and 265/65-17 which are 31" diameter.

Brent