Transmissions for dummies?

[ahoke]

New to the auto world and have been curious about the different options for 350 sbc transmission. Have a '70 GMC 3/4 ton I will be restoring once the snow melts and just curious why people swap the th350 out for a 700r4. Been trying to find a useful YouTube video but the search function these days is just overly targeted towards watching trash sponsored videos. The videos I do find are overly technical and glance over the simple aspects. In someone could list a few transmissions with a couple pros/cons would be much appreciated.

In short, I will most likely stick with the th350 that's already in the truck, but for daily driving/long distance trips, what's the ideal transmission? Not looking to win any races but the ability to comfortably cruise at 70mph sounds dreamy.

[dagnabbitt]

Originally Posted by ahoke

New to the auto world and have been curious about the different options for 350 sbc transmission. Have a '70 GMC 3/4 ton I will be restoring once the snow melts and just curious why people swap the th350 out for a 700r4. Been trying to find a useful YouTube video but the search function these days is just overly targeted towards watching trash sponsored videos. The videos I do find are overly technical and glance over the simple aspects. In someone could list a few transmissions with a couple pros/cons would be much appreciated.

In short, I will most likely stick with the th350 that's already in the truck, but for daily driving/long distance trips, what's the ideal transmission? Not looking to win any races but the ability to comfortably cruise at 70mph sounds dreamy.

There's nothing wrong with a TH350. The 700R4 has overdrive though, so it is a good transmission for highway speeds and fuel economy. But it is not suitable for towing, and, some people say, less durable overall. Others would point out that 700R4 failure is due to misuse, but I think it just depends upon what you use your truck for.

[kwmech]

A 3/4 ton with a T350? A little odd but not unheard of. Gear ratio and tire size come into play, and also the type of rear axle. 3.54s can be had for a D60. And of course what engine you have

[ahoke]

I have two 3/4 tons now. Both came stock with a 350 v8 and th350

[HO455]

The T350 3 speed non overdrive very common on later (post 68 as it was new in 69) 1/2 tons.

The T400 3 speed non overdrive was the step up from the 2 speed powerglide in 67 and in later years the heavy duty 3 speed automatic transmission

700R4 was GM's heavy duty 3 speed with overdrive automatic transmission. It was based on the T350. It got a bad reputation as a weak transmission. Some of it deserved (The early units) and some caused by improper operation. Lots of aftermarket support. Set up of the TV cable is critical or the transmission will self destruct quickly.

2004r was the light duty 3 speed with overdrive that came in cars only. It was the transmission installed in the Buick Grand Nationals and as such there is a good aftermarket and they can be modified to handle high powered engines, but have no tolerance for towing trailers so a poor choice for most trucks.

Both the T350 and the T400 can have a Gearvendor overdrive unit bolted to them giving them an overdrive.

I won't go I to the rest of the newer computer controlled transmissions that are out there that people use as I have no personal experience with them.

And if you want more in depth information click the link.

https://en.m.wikipedia.org/wiki/List..._transmissions

And the Gearvendor website.

https://gearvendors.com/index.html

There are lots of threads on all of these transmissions here on the site just do a search for the one your interested in. For example

http://67-72chevytrucks.com/vboard/s...d.php?t=701764

[Accelo]

700R4 was GM's heavy duty 3 speed with overdrive automatic transmission. It was based on the T400.
It based on the THM 350 not the THM 400. Some parts are even interchangeable between the two transmissions. GM gave them both the same torque ratings.

Lots of reasons to chose an Overdrive Transmission. One reason often overlooked it how much quieter it makes the cab. Typically they will reduce the engine rpm by one third.
Cheers

[HO455]

I apologize for any confusion my bad information caused. I have edited the post.

[connorm]

I am not sure of your specific truck, but I have a 1970 GMC 2500 (307, 4 speed, 4.10). This truck would rev out easily with factory size tires, 9.5x16.5 (31" OD), but once I put 285/75r16's (33" OD) it was much easier to keep up with traffic. I also started to realize that these trucks were meant to see 3k + RPM fairly often. with an auto trans you will have more slippage due to the torque converter, but you should have 3.73's as well. I would leave the TH350 and live with the RPM as this will be the cheapest, easiest, and most original for your restoration. Below are some different RPM and speeds for you to see on my truck, again your truck will vary slightly.

Right now: 307 to SM465 (1:1 4th gear) to 4:10 ratio to WORN 295's on the rear end
2800 rpm is 67 MPH
3000 rpm is 72 MPH
3200 rpm is 77 MPH

A "problem" that I see a lot of people having is getting these old trucks and thinking that they are not suitable for today's driving. They expect these old trucks to drive like today's vehicles and when they don't they start throwing new parts to get them to drive "right". At what point are you just driving a new car with an old body. I have no fear in driving my 70 GMC 3/4 ton 2 hours or more right now and for the last 3 years I have done a 3000ish mile road trip in a 1948 Ford with flat head and original 3 speed. These two vehicle drive like old cars but for the most part keep up with traffic and are not a hinderance. I believe that changing a gearset and matching tire size with that will get most people closer to what they want then a transmission.

[PbFut]

I agree with connorm. These 50 year old trucks will come close to matching todays engineering but at a fairly sizeable price. You get the gearing and right then the suspension is in need of attention. Now that you have the thing rolling down the road at the speed you want the stability needed. ok that fixed. Now you start chasing leaks. Get the leaks fixed, and well, the AC needs some tweaking. AC works, motor just doesn't feel like the new one's. OK Blue Print can fix that. Now you're rolling down the road pretty good. Need some tunes. OK the Ears a bleeding now. That's good. Humm paint could use some attention.

[connorm]

Originally Posted by PbFut

I agree with connorm. These 50 year old trucks will come close to matching todays engineering but at a fairly sizeable price. You get the gearing and right then the suspension is in need of attention. Now that you have the thing rolling down the road at the speed you want the stability needed. ok that fixed. Now you start chasing leaks. Get the leaks fixed, and well, the AC needs some tweaking. AC works, motor just doesn't feel like the new one's. OK Blue Print can fix that. Now you're rolling down the road pretty good. Need some tunes. OK the Ears a bleeding now. That's good. Humm paint could use some attention.

I want to clarify that I think that some of the charm of a 55 year old truck is that it is 55 year age. It doesn't have to be the smoothest quietest thing. Honestly I'm not real motivated to do anything to my truck right now because it works. It's rusted out and loud, but it does what I want it to do, and all for about $5,000. Its all factory suspension besides a factory sway bar was added and some fancy aftermarket shocks. I've fixed some leaks when they come up, the engine is under powered but I just deal with it, the speaker is a bluetooth JBL and the paint I just wax every once in awhile. Now to address the AC, also part of the charm of an old truck, wing windows.

I get more excited when I see a beat up truck being driven around by someone that truly enjoys it and has put sold blood, sweat, and tears in it then someone who bought a completely redone truck or has paid someone to do it and has no emotional connection to it. At the end of the day these trucks are hunks of iron, but I will NEVER sell mine because I can't replace the hours I have spent working on this with my dad. To me that feeling of driving an old truck and the memories with my dad in an OLD truck, not an old truck that drives like a new one.

[ahoke]

Thanks for the responses, fellas. I am totally onboard with keeping it stock, just making sure I do not have unrealistic expectations or unnecessary wear that will soon become a bigger issue after putting time into it.

Back story if anyone is interested: Picked up a '72 C20 last summer and have had a blast with it, even taking it to cut some loads of firewood. But I do not have a history or experience with working on vehicles. Sent my buddy a video of an oil leak shortly after buying it, and he suggested replacing the front seal. Next thing I know I am dropping the oil pan. (Again, no experience so I didn't know what a water pump was until I took a bolt out and antifreeze starting pouring out.) Wanted to know more about fixing these trucks and sought out an engine to rebuild myself. After not long on the classifieds it seemed smarter to just find another truck that needed the engine rebuilt. Found my 1970 C25 tucked away in a barn since 1998, no rust. Even has a dump bed on it.
Next project is to pull the engine and transmission (bought an overhaul kit for the TH350 that I'll be using).

[connorm]

Originally Posted by ahoke

Thanks for the responses, fellas. I am totally onboard with keeping it stock, just making sure I do not have unrealistic expectations or unnecessary wear that will soon become a bigger issue after putting time into it.

That truck is awesome. What you do with it is all up to you, but my opinion is that you should keep that truck stock-ish. I don't think that a TH350 is going to be a weak point that will cause issues. A C/20 is a great project that a lot of people overlook and just immediately convert to a short bed 1/2 ton. Good luck with your project and I encourage you to start a build thread in that section. I also recommend you get a factory chassis manual, you will be surprised how often you will refer to that.

I attached a picture of my 70 GMC C/2500, my dad's 71 Chevy C/20, and my mom's old 66 GMC 3/4 ton for inspiration. Please reach out with any questions.

[PbFut]

Jumping into the deep end of a pool while just learning to dog paddle is frightening, most come out alive. If you have a stick to it kind of personality, you will learn a lot and have a good feeling of accomplishment in the end. I find most of us on this board like to offer info to those willing to discuss and listen.