External Gear Reduction
 

Besides the fact some internals/gears can be switched out given the right circumstances for specific cars/hardware.

Knowing there have been External Gear Reducers(Overdrives) for very specific models in the 70's.

How hard/probability would it be to scroung one of these up and or Create one of these using the straight forwardness of DIY!?

Not to be selectable or variable, but a Full-time (overdrive) Possibly In-line with the drive shaft for ease of (use/construction/mounting). Basically giving you a revised (Final Drive) to your axle, without tearing apart into your tranny and elliminating the need to hunt down internal "compatible" hardware(for most don't exist)...

It would lead to a resurgence in interest in RWD among the EcoModder crowd, eh?

Gear Venders makes over drive units for most popular rear wheel drive vehicles and 4wd trucks.
They aren't cheap. For $2000 to $3000 are not economical due to the fact they tend to increase fuel milage by 2 to 3 mpg.
I would rather spend $500 or less doing a rear gear swap, it will deliver almost the same result.

As a teenager in the 1930's, my friend's dad extensively modified his Ford Model T and installed an entire 2nd transmission inline after the first one. You can see the 2nd transmission's gear shift in this photo (Model T's didn't have stick shifts)

http://forum.ecomodder.com/attachmen...1&d=1199032408

From: http://ecomodder.com/forum/showthrea...odel-t-92.html

This gave the car 7 forward speeds, and 5 reverse.

Junkyard transmissions are probably pretty cheap.

...but, with a rear axle swap, you (normally) cannot quickly switch "back" when heavy loads next to be hauled...although this is possible with some of the "2-speed" truck axles.

There was also an available 2-speed rear axle, the Ruckstell axle, one of the few aftermarket accessories approved by Henry Ford, but this one was more focused on lower gearing for uphill driving...

I know for some people it would make more sence to pay the big bucks to get more MPGs and retain towing ability, but since I dont tow heavy and dont want to spend $2k or $3k the rear gear swap works for me.

2-speed axles fell out of favor for some reason, I think its was because they were fairly inefficient at higher speeds.

The average new gearbox is fairly small. As such, you could install one inline of the driveshaft without using the tailshaft housing or bellhousing/clutch.

You can build planetary gear reducers cheap enough and have "custom" driveshafts welded to be able to DIY in/out (once the setup is complete) and have your normal gearing within an hour when you need it, just by swapping driveshafts and installing a planetary gearset inline with it.

Remember that the further you get from 1:1, the more losses you incur, and the more moving components you add, the more losses you incur.

In other words, making a 2:1 planetary set would not yield a 100% increase in torque to the road, because some small percent of the increase would be lost in the friction and fluid drag inside the new gear case.

Also worth noting... any gear reduction device is also a gear increasing device in reverse, for the most part. A small transfer case reversed might do exactly what you're asking for and provide an option to have two ranges by shifting from 2wd option to 4wd option.

I've often used 4L on my F150 (gone *sniff*) while the front hubs were unlocked to get a "crawling" gear for different reasons.

Imagine for a second that 4L gave me an drive speed of 1:0.5. Reversing the transfer case then, would have given me a 4L drive speed 150% greater than my input speed, or a drive ratio of .5:1 between the transmission output and the axle gears.

Compact-wise, nowadays a Subaru dual-range 5-speed stickshift is an interesting option.