203 underdrive?

[gmcman7202]

i am young, and undexperienced, but i am willing to learn and i have ideas.

ok so i have a 72 jimmy with a 400 sbc, the sm465/205 combo, with 3.73 gears, and 33x12" tires. i have a 203 sitting around, and i was thinking about making it a doubler, but i dont see me ever using it, as the truck already has alot of torque. I have crappy mpgs, and with these gas prices i have been looking at gear venders overdrive. i did some searching and came upon a site were they were talking about turning the t case around so the low was now .5:1 instead of the 2:1. i dont know if this is posible or a good idea, thats why i am posting here. i dont want to change the gears, caus then i would loose my low end for good, and the gear venders is to much money. i am just trying to use parts i have laying around, and still getting the results i am after.

thanks guys, ya all have been so much help so far.

 

[sled_dog]

custom adapters and shafts will add up quick. IT has been done and you could do it. You would have to have a custom reverse input shaft made, intermediate shaft between the 203 and 205, and custom adapters between the 203/205 and the Sm465 and 203. And I still don't think you will get what you may think you will for a MPG improvement.

 

[ntsqd]

I've never figured out how an OD could improve mileage if the rest of the gears were set up so that you could use a 1:1 as your top gear.

Any time you are turning gears you have frictional losses. Even with an OD.

If you set the ratios such that the engine turns the desired RPM at the intended vehicle speed, but are in a direct gear vs. an OD it should be slightly more efficient.