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tinman64
February 17th, 2013, 04:48 PM
I recently purchaced a 64 1000 with a 305,3speed,3.07 gears,I had never even seen a big block v6 before and am interested in keeping the 6 in it.I have heard all the negative reviews about the poor fuel milage these engines get,and was thinking about changing the tranny to a modern day O/D trans.The current set-up would put the engine@ about 2200rpm@ 60mph with a 28' tire,which isn't too bad.By changing trans to an O/D ( final drive about 2.15) the rpm @ 60mph would be about 1550rpm,this is right where the engine makes its most torque.Of course this is on paper.Has anyone changed their transmission to an O/D,and what were the results?Any help would be appreiciated as I would like to keep the original engine but not if it is,too expensive to drive.

Corts60
February 18th, 2013, 04:20 AM
When I put Stromberg 97's on my old '60 with 3.90 gears and a 3 speed, I was about 2900 RPM at 60. I never drove over 60 and I was still able to pull 11 MPG. That really is about what you'll get out of a big block Chevy as well.

One thing I am going to try and do on my next build is run a TBI set-up and some kind of overdrive tranny. I am hopeful that I could get about 15 MPG with that set-up and that would be just fine for as often as I drive my Suburban.

Funky61
February 18th, 2013, 04:27 AM
Here are some sites for reference that I used for this swap.

http://www.classicjunkyard.com/new_process_a833.htm

http://www.customclassictrucks.com/techarticles/0905cct_a883_four_speed_overdrive_transmission/viewall.html

raycow
February 18th, 2013, 05:02 AM
My truck originally came with a SM420 and I replaced that with a T-5 from a S-10. This was a fairly easy swap, but it wasn't a bolt-in. Here are the issues I needed to deal with:

1. I had to replace the clutch disc with a 14 spline disc from an Astrovan.

2. Transmission mounting holes had to be enlarged slightly to clear the stock 1/2" bolts. The lower two mounting holes on the T-5 had no threads, so I had to use longer bolts with nuts in those holes.

3. Input shaft and bearing retainer were too long. A commercial spacer plate is offered to solve this problem.
http://hamiltonintakes.com/products/s10-t-5-swap-adapter
My time is a lot cheaper than my money, so I shortened the T-5 shaft and retainer myself. Other people who have done the T-5 swap reported that they did not need to shorten anything, so you may not run into this problem. If your T-5 shaft is too long you will know immediately when you compare it with the one on your 3 speed.

4. I had to get a different driveshaft. Any driveshaft shop can make up the shaft you need, but I was able to find one in a salvage yard that worked fine after I changed the yoke. Here again, my time was cheaper than my money.

I swapped the T-5 only because my truck already had a floor shift. If it had been a 3 speed with a column shift, I would have used a factory SM318 3 speed with overdrive instead. This is very similar to the transmission you have in your truck now, except that the overdrive unit replaces the tail housing. The SM318 overdrive transmissions were offered on Chevy passenger cars and trucks from 55 to 65 and are not difficult to find.

The overdrive 3 speed is an easier swap than the T-5. You won't need to do anything with the clutch disc or input shaft. Depending on the transmission year, you may need to swap the driveshaft or driveshaft yoke, and that is all. If you get a car transmission you will need to use bolts with nuts in the lower mounting holes as with the T-5.

If you need more info, please feel free to ask.

Ray

tinman64
February 18th, 2013, 11:43 PM
Thanks for the input,I was thinking more like an automatic trans,does'nt sound like anyone out there has experimented with an automatic,I think it might be worth a try.Would there be any balancing issues when switching from a flywheel to a converter and flex plate.Any experts out there?

Funky61
February 19th, 2013, 02:47 AM
Here's the info for starters...


http://www.6066gmcguy.org/SM420.htm

melmashman
March 27th, 2013, 03:11 AM
My 1965 GMC Truck/Camper which I just inherited from Dottie, my step-mom, has a 305eV6. She recently passed away, but was able to obtain new motor mounts for it which it badly needs. I am planning to have them installed in May near Prescott, Arizona.
I got to thinking that the clutch must be pretty old by now with over 100K miles and I thought it might be a good time to put a new one in.
Then I got to do more thinking and the one thing I disliked about the vehicle was its lack of an overdrive. I took it to Alaska and maxed out at 50mph without severely overworking the wonderful engine. So now I am thinking I might switch out the 3 on the column to a used factory SM318 3 speed with overdrive Ray mentioned. I suspect the rear-end is geared too low, but did not want to mess with it now.
I don't know if I can get all the ducks in a row to get it done at one time (it would be done by a truck garage - not me), but I wondered if anyone could help me with sourcing the parts.
Also, do you think my thinking is correct or nuts?

BillT
March 27th, 2013, 04:34 AM
I super-tuned my '66 GMC 1000 with a 305 V-6 just before a long trip years ago and I was getting a consistent 15 1/2 MPG out of it, highway.

I would have loved to have a Factory OD with my 3-Speed back then as I would love to have one today for my '72 C-10, but I always found them very hard to find.

I checked into aftermarkets and found them to be pretty expensive.

I went the inexpensive route with my '72 and put tall tires on the rear. I've got 265/75R/15's on the rear and a matching set of 235's on the front.

It did make a notice-able difference.

Bill

raycow
March 27th, 2013, 08:07 PM
Melmashman, you might want to look into this one before it gets away. I didn't realize they were getting hard to find.
http://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=925247

Ray

huntaaar
July 17th, 2013, 06:41 PM
I super-tuned my '66 GMC 1000 with a 305 V-6 just before a long trip years ago and I was getting a consistent 15 1/2 MPG out of it, highway.

I would have loved to have a Factory OD with my 3-Speed back then as I would love to have one today for my '72 C-10, but I always found them very hard to find.

I checked into aftermarkets and found them to be pretty expensive.

I went the inexpensive route with my '72 and put tall tires on the rear. I've got 265/75R/15's on the rear and a matching set of 235's on the front.

It did make a notice-able difference.

Bill


What do you mean by "super tuned" 15mpg is my goal (im at ten now up from 7). tell me more about the set up you had. thanks.