Need suggestions for transmission and rear end

[rally]

Hey everyone,

I understand there's a lot of factors to consider in this, and a lot of people ask the same question, but I wanted to start a new thread so I can ask my own questions as I learn. I'm not very knowledgeable in what my options are, but here's what I have and what I'm aiming for (and looking for help on):

I currently have a 350 engine, 350 transmission, and a 12 bolt rear end (not sure of any ratios).

I don't plan on drag racing the truck, but definitely want to be able to come off the line a lot quicker than what I do now (the response feels kind of slow). So more torque would be nice.

On the freeway, I'd like to get the RPMs down, but I don't have a tach, so I don't know what they are. Maybe they're in a good range, but they sound kind of high at 70-75 MPH (although, I'm running 18" front tires and 20" rear tires, so the speedo is off a bit. I'm probably going a little faster than that). Traffic in California is usually pretty fast, cruising around 80 or so, at least in the Southern half. I'm not looking to fly through traffic, but saving MPG while keeping up with traffic would be nice.

What I was thinking was swapping out the 350 tranny with a 700r4, hoping that would allow me to drop the RPMs on the freeway and save a little on the gas mileage. Maybe switching out the rear end to a different ratio would help, too, but I don't know where I'm at or where I should go with that.

What would you guys add/change for this setup to get a truck that's quicker off the line, but still decent on freeway gas mileage? Maybe the current configuration is good enough with a little tweaking here and there?

[rally]

I guess I should mention it's not going to be a daily driver, but I do plan to take it out every week or two because...well, it's just fun to drive.

[67ChevyRedneck]

It sounds like you have a 3.08 rear end now (was common on automatic trucks). This will be about the best non-OD combo for fuel mileage. Yeah, not super fast, but like you said, decent highway rpms, but slow.

If you want speed/fuel mileage, one of the best bang for the buck combo's is a 3.73 and 700R4. Don't expect a fuel mileage miracle, it will still probably only get 11-12mpg around town and 15-16 on the highway. Probably around 15 if you like going 75mph.

I would caution using 3.08 gears with the 700R for better mileage, a lot of members here got better mileage when they went to a 3.42 or 3.73 because their motor was luggin in OD with the high 3.08 gears.

In your case, if you only drive it a couple times a month, and the rear and trans are good... I would leave it alone for another 6 months - year to see if you really want to change it.

[rally]

Originally Posted by 67ChevyRedneck

It sounds like you have a 3.08 rear end now (was common on automatic trucks). This will be about the best non-OD combo for fuel mileage. Yeah, not super fast, but like you said, decent highway rpms, but slow.

If you want speed/fuel mileage, one of the best bang for the buck combo's is a 3.73 and 700R4. Don't expect a fuel mileage miracle, it will still probably only get 11-12mpg around town and 15-16 on the highway. Probably around 15 if you like going 75mph.

I would caution using 3.08 gears with the 700R for better mileage, a lot of members here got better mileage when they went to a 3.42 or 3.73 because their motor was luggin in OD with the high 3.08 gears.

In your case, if you only drive it a couple times a month, and the rear and trans are good... I would leave it alone for another 6 months - year to see if you really want to change it.

Thanks for the info/suggestion 67ChevyRedneck.

I think you are probably right about having 3.08 gears on my truck from what I was able to read up on and understand. It sounds like 3.73 with a 700r4 is a good combo with a 350 engine. I don't have the money to drop on everything right this second, so I'll keep saving up and evaluating where I am for mpg vs fun.

Is a 700r4 the only option for a 350 engine on our trucks if I want an overdrive?

One person I spoke with a few weeks back mentioned he had a TH350 with an electronic overdrive...I didn't think the TH350 had an overdrive option, and I'm not really sure how that works if that is a valid option. Can you shed some light on that?

[Wide Open]

Originally Posted by rally

Thanks for the info/suggestion 67ChevyRedneck.

I think you are probably right about having 3.08 gears on my truck from what I was able to read up on and understand. It sounds like 3.73 with a 700r4 is a good combo with a 350 engine. I don't have the money to drop on everything right this second, so I'll keep saving up and evaluating where I am for mpg vs fun.

Is a 700r4 the only option for a 350 engine on our trucks if I want an overdrive?

One person I spoke with a few weeks back mentioned he had a TH350 with an electronic overdrive...I didn't think the TH350 had an overdrive option, and I'm not really sure how that works if that is a valid option. Can you shed some light on that?

Maybe they were referring to the 4L60E (electronic version of the 700R4) or perhaps the 4L80E which is sometimes referred to as a TH400 with an electronic overdrive.

Both require electronic controllers to operate. They default to second gear only with no electronics. I've read of some people doing full manual valve body conversions on them but can't speak to that myself.

[67ChevyRedneck]

In terms of automatics, you could also use a 2004R. Some people like those better. But no, really no other "affordable" options. The 4L60E will need a stand alone computer to run with a 350 and they're $$. A gear vendors overdrive will work with your TH350, but again, $$$.

If you want a manual, you have those options as well.

[Wide Open]

Originally Posted by 67ChevyRedneck

In terms of automatics, you could also use a 2004R. Some people like those better. But no, really no other "affordable" options. The 4L60E will need a stand alone computer to run with a 350 and they're $$. A gear vendors overdrive will work with your TH350, but again, $$$.

If you want a manual, you have those options as well.

Did the 200's come with the SBC bell housing bolt pattern or were they BOP? I can't remember.

[Mike C]

"Put your money where your mouth is", and with that in mind, I am getting a TH200 4R from Bowtie Overdrives. I dislike immensely the 1-2 gear ratio split on the TH700 trans and choose not to pay the absurd amount it would cost to do a TCI 4l80 based 6 speed or one of the GM trans.

TH200 4R and 3.73 is where I've decided the best fit for my is behind the 427 I'm putting together. (Although I may go 3.90 when push comes to shove since I'll be buying gears) The 2.74 first gear gives you an overall first gear of 10.2 with the 3.7's which is the same as a TH350 with a 4.0 gear. The .67 overdrive knocks the 3.73 down to a 2.50 for highway cruising.

Best of both worlds with 4:1 launch gear effectively and 2.50:1 cruise gear. Should be able to get the Stage III (rated to 500 hp) in the vehicle including any needed driveshaft mods for $3000.

The only way a TH700R4 is better than the TH200 4R is in price. You can get a takeout from an existing vehicle easier, but once you build to take any power the price is pretty much the same.

[hoss67]

I put 200r4 in my 67. Replaced the th350. Didn't have to change the driveshaft length. Had 308 with the th350. Changed to 3.73 with the 200r4. Best upgrade.

[Wide Open]

Originally Posted by hoss67

I put 200r4 in my 67. Replaced the th350. Didn't have to change the driveshaft length. Had 308 with the th350. Changed to 3.73 with the 200r4. Best upgrade.

So the 200's will bolt right up to the SBC's? No adapter needed?

[hoss67]

Originally Posted by Wide Open

So the 200's will bolt right up to the SBC's? No adapter needed?

Look for one that came out of 82-88 monte carlo with 305. Bolts right up.

[Wide Open]

Good to know. Thanks!

[hoss67]

Originally Posted by Wide Open

Good to know. Thanks!

No problem. They are getting hard to find. Good luck.

[rally]

200R4? I'll read up on that. I'm not familiar with that at all.

As far as, say, a 4L60E and a standalone computer, how would you hook that up? Just bolt an ECU (I'm assuming the ECU is the standalone computer?) to the firewall and just wire it to the engine and transmission?

And an SBC with 3.73 gears sounds like a good start. What about changing out the gears to 3.73 and running with the 350 transmission (until I save up enough to buy a different transmission)? Would that be a good place to start, or would the 3.73 be a bad combo with the TH350?

[hoss67]

Originally Posted by rally

200R4? I'll read up on that. I'm not familiar with that at all.

As far as, say, a 4L60E and a standalone computer, how would you hook that up? Just bolt an ECU (I'm assuming the ECU is the standalone computer?) to the firewall and just wire it to the engine and transmission?

And an SBC with 3.73 gears sounds like a good start. What about changing out the gears to 3.73 and running with the 350 transmission (until I save up enough to buy a different transmission)? Would that be a good place to start, or would the 3.73 be a bad combo with the TH350?

Well I can answer part of your question. I had 305/50r20 on my 67 with 3.73 right before the swap to 200r4. Wasn't to bad on hwy. But if you run a smaller tire the rpm will be very high at hwy speed.

[Mike C]

Is cost important? How do you expect to drive the truck? If you don't mind the $1000+ Dollar cost for the 4L80E transmission controller and the cost of the trans itself it is a good choice for a high output engine combo. But to get the low gear multiplication of the TH200 4R or the TH700R4 you need to run 4.10 gears so the overdrive not quite as big a benefit.

If you are only bee-bopping around town with your truck only doing highway speeds for short distances, the 3.73 will be fine. Especially if you have a near 30" tall tire.

The TH200 was a late 70's light duty trans or "metric" trans they called it. They added the 4th gear and made it a TH200 4R. I was in G bodies for the most part (Malibus, Cutlasses, Regals, etc. including El Caminos.) But it really found it's place in the Buick Grand National. The GN guys figured out how to get it to survive behind 600+hp and 100 lb/ft motors.

Junk yard shopping for 200's with the multi-engine bellhousing pattern (it is cast for BOP and Chevy) can be hit or miss because it was relatively low volume compared to the 700's that were made in the millions in trucks.

But if you are buying an aftermarket, built up transmission, the TH200 4R is the better choice IMO. It can be made to take a stout street motor and doesn't have the extreme 1-2 split the 700 has. It's drawback again is that it is not common locally and needs to be built to take any real power. Figure $3000 to get it into your truck depending on options and other mods you make. It does NOT require driveshaft mods to replace a TH350, just moving the crossmember back to the TH400 position.