Best Tcase/tranny combo for mileage?

[77K10Silverado]

I've got a 77 K10 with a 350/TH350 now. What my best swap option? I can do driveshafts too. What's worked for you guys??

[JIMs70GMC]

swap in a 700R4/NP208 combo

You'll get overdrive and the 208 has a lower low range then your 203. new driveshafts will be a must, probably get them from the donor truck.

[77K10Silverado]

Jim that's what I was thinking. Does anyone happen to know what years and models they were using that combo in? I have a pretty solid u pluck it yard near by that I may take a look for.

[JIMs70GMC]

I believe you want a combo from an 84 or newer, i believe the earlier 700r4s had some issues. 84-87 and 84-91 (K5&Suburban). I am sure someone else will correct me.

[77K10Silverado]

Heck yeah. Thanks for the info on that. Is the 2wd 700r4 different from the 4wd version? Or is that what you meant by the get a combo?

[gmachinz]

Get some aluminum shafts from a newer truck and have a new one balanced for your truck. Then, go with a fairly narrow tire/rim package for less rolling resistance-and use a more universal tread pattern. Wide, nobby tires kill mileage and put a lot of stress on the steering components.

[andrewmp6]

82 and newer had 700r4 1/2 ton and 3/4 ton.Yes the 2wd and 4x4 trans are always different.You can turn a 2wd in to a 4x4 trans but it takes pretty much a rebuild to get it changed over.The 700r4 4x4 came with a np208 t case on it those are ok on a stock truck,If you want more of a hard core off roader then looking in to a np205 which takes a swap kit to use on the 700r4 4x4 trans.

[77K10Silverado]

Thanks Andrew! No hard core-ing here! lol I'll go do some looking in the yard and see what I can't find. If it's reasonably priced, i'll do the swap!

[andrewmp6]

To tell what trans your looking at look at the trans pan and count the bolts.The 700r4 has 15 bolts and on the t case the part pointing to the back of the truck will have a round badge on it.On that badge it should say np208 or np 208 spaced out but means the same thing.But since your swapping it in a older truck you might want to grab the steering column the shifter linkage for the trans,The trans crossmember the t case shifter and linkage and both driveshafts if the truck is the same wheel base as yours.If its not the same wheel base grab the front and find the rear later or have it made.

[77K10Silverado]

Will do! This sounds like it might start to get expensive... :/ I'm def going to look into it and at least see what my local yard has to offer. What would one pay for the tranny/Tcase and a set of shafts?

[andrewmp6]

Prices very junkyard to junkyard you could spend 100 bucks to a grand for all of that.If its like pull a part they are super cheap if its a small old junkyard they might ask more for the parts.Worse case see if you can find a parts truck cheap thats rusted out bad or needs a ton of work.

[jjzepplin]

I found my 700r4 for $50 in a scrap pile at a junk yard-$300 for a B-M rebuild kit and $20 for a rebuild video. $120 for a swap over shaft to fit the NP205 and I have a bullet proof unit for the more than a decade. My first trans rebuild. I remember that you can find the build date on the serial # and you don't want one 1986 or older. Mine is an 89. I run 4.10s and love hearing that 4th gear kick in and then the lock-up make an even sweeter humm.

[lindstromjd]

You forget that a HUGE contributor to fuel economy is in how you drive. If you add an overdrive into the equation, you better be doing a lot of sustained, higher-RPM driving. If you do a lot of in-town stuff or sustained low-RPM driving, then you'll probably get worse fuel mileage. The other piece of the puzzle is in your rear end. You have to have the right combo to get good fuel mileage... something the GM techs didn't take into consideration when they gave trucks a 2.72 rear end and a 700r4, and then people wondered why they got such horrible fuel mileage when they weren't on the interstate.

[OutlawDrifter]

a well tuned carb, good plugs/wires/distributor, and a light foot can do wonders. my old k10 could do 15mpg if i drove it nice. it was a similar setup to yours.(350/350/203, 3.42 gears)

[1976gmc20]

Originally Posted by OutlawDrifter

a well tuned carb, good plugs/wires/distributor, and a light foot can do wonders. my old k10 could do 15mpg if i drove it nice. it was a similar setup to yours.(350/350/203, 3.42 gears)

My 1980 K-10 (350/465/205) would get 18 mpg easy on the highway if you didn't drive too fast, and 15mpg at 80mph!

I don't know what the axle ratios were, but it was tall! You had to run up to about 35mph before shifting into high gear, so it was like a four speed overdrive. With the granny low it was still fine off road.