1970 GMC Automatic Transmission Upgrade

[Bob Kelly]

10 miles per gallon is starting to wear me down! Trying to decide of a change to a TH700 or TH200 would appreciably increase my mileage. Here's the scoop:
1970 GMC 2500 400CI 3.57 Rear End
My questions are:
(1) does some one sell a "soup to nuts" conversion kit i.e. transmission, shifter kit, directions, transmission cooler, etc. I'm looking for one stop shopping for a plug and play conversion kit.
(2) will the fuel savings make it worthwhile to do the conversion.
Thanks for any help/direction you may give me, Bob

[leftybass209]

Question 1: Early classic enterprises sells the closest thing to a kit for a 700r4. However, no there's no kit that I know of, containing transmission lines and an external cooler.
Question 2: It depends on how much you drive it. It will cost 3k for brand new or rebuilt. If you drive on the highway often, for thousands of miles, you will eventually recoupe your money. You have to break it down to cost of gas and miles driven in overdrive, vs the gain of mpg. There's a lot of variables for an accurate estimate.

[Already Gone]

I've been wanting a 700R4 for years for my BB 72. Seeing I have only driven it 5000 miles in the 12 years I've owned it I cant justify the expense for exactly the reasons stated above. A 20% increase in gas mileage from 12/13 mpg to 15/16 mpg wont save you much unless you are driving mega miles in a year.

[67ChevyRedneck]

I know someone will chime in with 20mpg, but I've never gotten better than 16.5mpg on the highway with my truck. It has a 350/700R/3.73. I average 12-13mpg with it (mostly around town cruising.) I used to get 11-12 with the old 3spd and it screamed at me over 55mph. I average 14mpg with my fuel injected 5.3L 2000 Silverado.

Is it an auto or manual now? Bowtie Overdrives provides easy to install kits for the TV cable and converter lock up, very important for the life of the transmission, some great reading here:

http://www.bowtieoverdrives.com/index.shtml

I did mine more for the drivability the OD provides. 2300rmps @ 70mph is nice!

[Tom Vogel]

700r4 might be ok for a c-10, but I would get a Gear Vendors for a C20. It would bolt right to the t400 and effectively gice you 6 speeds... and you Wont break it.

[Longhorn 70]

I have a 700R4 in my C20. It was done in 1997 and cost about $1200 for everything then. I didn't do it so much for the fuel savings. It was more for the don't make the engine rev so high going down the freeway. I MUCH rather hear it at 2000RPM than 3200RPM on a long cruise. That's just me though. I also like the easier take off when I drive. I tend to drive easy and the LOW first gear helps. It also helps boil the tires if I want to...not that I would on the street...that's illegal.

[Bob Kelly]

WOW! Thanks for the multiple in-depth answers to my conversion for gas mileage questions! The answer in a nut shell is there is no way I can recoup the cost of the conversion in my lifetime. The conversion came to a head because my son Chris (high school senior) is driving the truck to school daily. He will graduate in June and the truck will revert to its 2500 miles a year usage.

The truck is automatic with dual glass packs, cruise control, and Edelbrock carb. Some factory options are brake boost, power steering, air conditioning, and extended bed.

About 6 times a year I tow a trailer mounted 22 horse power 1916 Stover Hit & Miss engine to shows (5,000 pounds). I always say the second most exciting thing you can do with the truck towing the Stover is go up the Cuesta Grade!

Thanks again for all the help, Bob

[67ChevyRedneck]

That's a great looking Longhorn you got there!

[mrein3]

Originally Posted by Bob Kelly

WOW! Thanks for the multiple in-depth answers to my conversion for gas mileage questions! The answer in a nut shell is there is no way I can recoup the cost of the conversion in my lifetime. The conversion came to a head because my son Chris (high school senior) is driving the truck to school daily. He will graduate in June and the truck will revert to its 2500 miles a year usage.

The truck is automatic with dual glass packs, cruise control, and Edelbrock carb. Some factory options are brake boost, power steering, air conditioning, and extended bed.

About 6 times a year I tow a trailer mounted 22 horse power 1916 Stover Hit & Miss engine to shows (5,000 pounds). I always say the second most exciting thing you can do with the truck towing the Stover is go up the Cuesta Grade!

Thanks again for all the help, Bob

One more note.
For what its worth I wanted to convert my 3-speed manual, 4x4 to an automatic transmission. I, like others have posted, did the math and determined that there was NO WAY it makes economic sense to go to the extra expense of a 700r4 even when doing what I was doing. (Swapping from something to an auto vs. your taking out a perfectly good auto for another).

Gas was cheap when I did my swap 12 years ago. Gas was $1 - $1.25 per gallon. I even went so far as to project gas at $2 and $3 per gallon and is still didn't "save" money.

All that said, like Longhorn 70 points out, I wish I would have gone the 700r4 route for the simple fact that you can't hold a conversation with somebody in the cab when I'm rolling down the slab at 70 mph. You can't hear your cell phone - which is probably a good thing. And it probably isn't doing my 383 any good with those high r's for a long time.

Oh and if you do swap, when it comes time to pull something, make sure you tow in "Drive" or 3rd. You'll kill that transmission pulling in OD no matter what the rebuilders out there tell you.

[Cali72K20]

This is why this site RULES!
I have the exact same issue, I first wanted to do a 700R4 swap, then read that it is not a cheap solution.
Then i saw this on here:
http://www.gearvendors.com/4x4gm3s.html
Once I saw the price, not an option.
Now I know I will just re-gear to 3:08's and call it done. Mine is a garage queen that only gets driven when it is nice out or to local car shows.
I will have to get new carriers for the new cogs.
I'm trying to keep it as stock as possible, but 4:10's suck on the highway!

[Cali72K20]

It is a small world Bob Kelly, nice truck, I don't think I have seen your's here in town.

[67ChevyRedneck]

Originally Posted by Cali72K20

Now I know I will just re-gear to 3:08's and call it done. Mine is a garage queen that only gets driven when it is nice out or to local car shows.
I will have to get new carriers for the new cogs.
I'm trying to keep it as stock as possible, but 4:10's suck on the highway!

That's actually about the best option for weekend cruisers. I did mine because I was putting 6-8K/year on it and doing a lot of highway driving.

I bought a 65 289 Mustang a year ago. Before I had purchased it I was planning on an OD (5 speed manual) but after putting over 3,500 miles on it, it's fine the way it is. It has a high geared 2.8 rear end. Keeps the RPM's relatively low on the highway and still gets about 15mpg.

It also doesn't *have* to be expensive. From memory, this is my breakdown:

~$800 for local rebuilt 700R (this was all in 2006)
~$200 for Bowtie Overdrives TVmadeEZ and TC lock up kits.
~$100 for new lines and aftermarket cooler.
~$50 for shortened and balanced driveshaft (I've been told this was stupid cheap? I must have caught the guy on a good day?)
~$100 for used Lokar shifter (another steal but a factory auto shift column will work, my dad even rigged up a 3spd column to work with an auto, you just have to count the gears )

If you already have an auto in your truck and it needs to be rebuilt... it's almost kind of a "why not?"

[MARKDTN]

Couple of things.
1-I have a 700R4 in my '83 K20. I have gone a lot of miles towing with it. The original lasted 70k miles before the pump died. Then I got it built and broke something inside about 10k later. I got a new later core (very important!) and I have about 188K now.

2-Fuel injection goes hand-in-hand with mileage increases with overdrive. Same '83 K20 got 14 or so City and 17 Hwy since I got it at 42k miles from my uncle. Swapped to Tuned Port fuel injection and went to 17 city and over 20 hwy.

3-Nobody has discussed the cost factor of wear and tear on the engine. If you aren't running 3500 rpm on the interstate but instead are at 2450 rpm (.7 overdrive), there is a lot less heat being generated and a lot less wear on the engine, radiator, and fan. That number is hard to attach a price to, but a good rebuild will cost about $2000 and a couple of weekends work to take it out and put it in plus downtime in between. Something else to consider.

[Bob Kelly]

This probably doesn't go here but this is a video of the 5000 pound Stover bouncing my 1970 GMC around. The Stover is 1007 cubic inches-one cylinder.

https://www.youtube.com/watch?v=dnURYFkV3R4

[bbcmudtruck]

If you hunt around on craigslist, you can find a gear vendors for half of what they cost new. Easy to hook up and will only require you to shorten your drive shaft. Theres basically nothing to them internally, thats why they are considered bullet proof. The one and only trick is finding a GV that has the right adaptor to your trans... I came into one cheap however I have to hunt down an adaptor for the th400, which cost a fella around $500 bucks new. Craigslist will eventually turn one up, just gotta be patient. As far as 700r4s goes, I'll never own another one again! I had one in an 84 2wd that was built and fresh. It lasted a whopping 15,000 miles before it started throwing fits, and it was behind a mild 454. Absolutely noting fancy whatsoever. Seems like they are very hit and miss on longevity. Plus, if you eat one, its twice as much money to have it rebuilt than a 350 or 400. My vote goes to Gear Vendors!