Why not have a REAL overdrive 5th gear?

Hi. I'm new here.

I bought my first Tacoma from my cousin in preparation for moving to teach in Alaska. Needed the four wheel drive. He sold me his well-cared for 1998 Xtracab with a V6 and 31x10.5 tires. Manual transmission.

Goodness! I've never owned such a vehicle. It's big to me, but, I'm told, will prove puny by Alaska standards. I need to change some things on it to make it as safe a vehicle as I can. But one thing that came to mind was saving in gas mileage. I found a handy speedometer converter gave a range for the trade-offs when you change the highest gear ratio.

And I noticed that my truck's stock 5th speed gear ration was a 1:4.11. In order to save fuel, I suppose I could make that overdrive fifth gear into a REAL overdrive and change it to something smaller, like a 3.42 gear ratio. Here's my question(s): Why wouldn't I do that? If it's feasible to do for a highway cruising gear, why doesn't everyone do that?


Here's what I'm starting to work with:

 

4.1:1 is your *final* gear ratio (at the differential), not the 5th gear ratio.

With a 1998 V6 4X4 5-speed (R150) you have a 5th gear ratio of 0.838:1.

 

Thanks EnolaG– details I don't know about...

...but is it possible to change it to something with a little less ooompf, but lower rpm, hence save gas? How difficult would that be?

 

In theory, it's do-able by swapping to a higher (= lower ratio number) final gear ratio.

However, there are two 'gotcha's' ...

(1) The higher the gearing, the worse your low-speed 'grunt' capability will be ...

(2) You'd have to do it twice (you've got 2 differentials, and they really need to be geared to match ...).

 

Just put on a little bit taller tire.