1968 C20 transmission swap advice

[Ol' Chevy]

Howdy,
I have a 1968 C20 4x4 with a 4-speed transmission. The diff's are geared at 4.56. It really screams going down the highway, and the gas mileage is terrible. I was considering re-gearing to 4.10, thinking that may at least quiet her down on the highway. I am now wondering if I can achieve the same result by swapping out the 4 speed transmission for a 5 speed. Has anyone done this? Did you gain fuel mileage? Lose power? Any input will be helpful. Thanks.

[JIMs70GMC]

The NV4500 is the good swap candidate, 2 different chevy versions, one has a lower first gear. Both have OD. Both will work. Used in 90's trucks. I have not done it myself.

[newstedcomeback]

The NV4500 would be the best choice, However it is not a very simple conversion. there is alot involved such as the clutch linkage, drive shaft length, transfer case adapter, finding a 4x4 version of the NV4500. the transfer case that came with the NV4500 are all for the drivers side front diff so you would need a conversion adapter for the NP205. alot of time and $ will be involved, but in the end you will be very happy.

as far as re-gearing the rear end, you would have to do the front axle as well to Keep the gears the same for 4x4. If you cant do the gear setup yourself, It can be pretty expensive.

Overdrive is a wonderful thing. I have a 700r4 on my truck, right now its is in the process of being put back together, but when I went from a th350 to a 700r4 it was a great upgrade. have you considered swapping to an automatic? because you can find 700r4's all day at junkyards, many of them have np208's already attached which is set up for the pass side front diff. that combo has a great low range and Overdrive. prettymuch a win-win. that would be a much simpler conversion in my opinion. 700r4's are not the strongest, but they were used in 3/4 ton trucks and suburbans and the np208 was used in 1/2 3/4 and 1 ton trucks. my buddy's CUCV has one in it.

[jeffahart]

I can tell you I had 4.10 with the SM465 and 33s and loved the setup. I now have 4.56 with 33s and it's screaming metal now for sure. But going to 4.10 is the wrong side of the carrier break and you will be into it for some money. As long as you are spending the bucks, might be better to go a bit taller gears 3.73 than 4.10 and adjust down with tires. And I do think the 4.56 gears are more noisy, they seem to bring out the harmonics in everything. If you are touching the transmission I would go automatic. What more could you want for compound reduction and overdrive? I feel like you have the best manual truck transmission you can get. And of course it all depends... on... What you use the truck for... your skill level for conversions.... your wallet depth... and level of PATIENCE!

[obijuan]

put some 38's on it.

['63GENIII]

While the 4500 isn't a real straight forward swap depending on the transfer case combo that you have, it really is a cool set up. I have 4:11's with 35" tires and the truck used to howl down the freeway at 70 - 75. It just kid of lopes along comfortably now. I went to the 4500 from a TH400. I also had a divorced T221 at the time. I now have a divorced NP205. Huge difference. The truck seems to run around 2200 - 2400 where as it used to run around 2900 - 3000 rpm.