I been still working out this issue myself. I thought about swapping in a 6-speed manual(because it has lower 2nd and 3rd gear ratios) and I have considered changing my 3.73

Currently I do this for speed:

Speed/RPM Calculator - stock

I been looking at 3.54 rear in conjunction with the 6-speed.

Speed/RPM Calculator - proposed modifications

The difference is... I often drive, 25, 35,45,60,and 75.

25 - 300/600 rpm drop depending on road conditions
35 - 100/500 rpm drop depending on road conditions
45 - 100 rpm drop
60 - 100 rpm drop
75 - 100 rpm drop

As you can see that my lower gears I have gained the freedom to switch between two gears to dictate whether I get a 300 or 600 rpm drop in rpm(25 mph), where you are stuck with what the automatic chooses.

The consequences is that now you have to work harder to drive your manual than had you have a free hand and mind by driving the automatic.

Your towing capacity will require more demand on the gear you are in since I dropped the final drive ratio. But you have power on demand down below and mileage up on top.

This is something I will put on the card table when my 4-speed gives out. Then the cost of replacing everything is in my favor since I have no choice to spend money on it if I wish to see it on the road again.

 

I won't deny you the 6.7 is more efficient. But it's gain is very small; that's my point.

I appreciate you taking the time to explain the multi-injection efficiency.

 

Granted you need new tires but I'm interested to see if you recoup the cost of the new 20" wheels with the little if any gain you see in fuel mileage. You are making a slight change in your end gear ratio but will it be enough to make that much differance? I doubt it. No offense but I think the driving force here is the desire for the 20" wheels and you're trying to justify it with the increase in fuel mileage which won't happen. But again if you want the 20" wheels. go for it, the little differance in those tire sizes won't matter.

 

Back in 1975 or '76, I had a 454 Chevy Suburban which got very lousy gas mileage.

I thought that I would just install a Gear Vendors overdrive and solve the problem.
a bit of mathematics proved that was pure folly,

it would take me driving 100,000 miles just to break even on the cost to install it, versus the $aving in fuel costs....

minor changes in rear end ratios, or tire sizes is folly if you want better MPG.

 

Bigredtoy, did you realize this is almost a 13 year old thread?

 

Maybe by now he's recouped the money