stay TH350 or convert to 700r4

[ducksfootball82]

I am debating between switching from a TH350 to a 700r4. Based on what I have found so far it's as clear as mud if it's a good idea. I hear people love their 700r4's and other say to stay away from them.

Vehicle is a 72 3/4 ton, 4.10's, being a daily driver mostly in town with the occasional towing of a travel trailer and occasional road trip. I recently bought it and drove it up from Oregon to Idaho getting only 7.2mpg staying out of the secondaries as much as I could, cruising about 55. In search of better gas mileage. Any help is appreciated.

[60sIron]

Thought about this often. I feel it comes down to expense = results. Realistically what kind of increase in gas mileage do you feel you'd get. 20%? Generous in my opinion. That still puts you below 10 mpg. Being you already have a automatic it'll save you $ in a swap. Lets say it'll cost $1000. How much are you going to drive to make that up and then start saving on fuel? I won't do it. Now if you were having problems with your 350, then yes I would always upgrade with an eye on making it more driver friendly. But that doesn't sound like that's your case. Just my 2 cents.......

[LockDoc]

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I don't know if it's coil or leaf rear but I would try to find a complete 3.73 rear end for it and swap it out.

LockDoc

[bntmotorcycles]

Like many I have the same problem with mileage. I have a 4spd. though. I found a 3:54 diff. for mine and I'm going to try that and see what happens. Most of my driving is hwy. or around town, not much pulling. Hoping for 14 or so.

[AzDon]

Too many RPMs for the highway with your current setup..... I'd do BOTH the 700r4 conversion AND a more highway friendly diff ratio....
My 68 Burb runs a 700r4 and 3;07 gears..... At 75, it purrs along at 1600 RPM and gets over 14mpg..... around 10-12mpg around town.....Pretty good for a 350 with a Holley Double-pumper!

[LockDoc]

Originally Posted by AzDon

Too many RPMs for the highway with your current setup..... I'd do BOTH the 700r4 conversion AND a more highway friendly diff ratio....
My 68 Burb runs a 700r4 and 3;07 gears..... At 75, it purrs along at 1600 RPM and gets over 14mpg..... around 10-12mpg around town.....Pretty good for a 350 with a Holley Double-pumper!

I had a completely different experience with my 2004r and the 3.07. The 3.07 made my transmission overheat in overdrive and puke all of the fluid out of it. Once I switched to the 3.73's no more problems. I got 16mpg driving from central Iowa to Kansas City when I drove the Burb to the All Truck Nationals.

LockDoc

[ducksfootball82]

What modifications are all required? I know the r4 is slightly longer and may require the driveline to be shortened, would the front one have to be lengthened? Could I move the motor mounts forward in the engine compartment to offset the additional length of the r4? What about crossmember and shift linkages? Sorry for all the questions, just want to be thorough before a decision. I was also thinking a monster transmissions r4 for the swap. Would that be a good one or would you recommend someone else?

[Average Joe]

Have you considered a 4l80 swap with the stand alone controller? It is a much more durable tranny plus no more tv cable to mess with.

[imjeff]

Neither...I went with a T-400. It won't help the mileage, but it won't break either.

[ducksfootball82]

I've thought of both the TH400 and the 4l80, I've heard great things on the 4l80 but want to stay away from the electronics. And for the T-400 haven't looked into much, would it bolt right up or would modifications be needed? If I did a th350 but beefed it up a bit I figured it would be fine. Thoughts?

[60sIron]

Ducks have you done anything like this before? Any swap involves work and $. In your original post it sounded like you wanted to impove your gas mileage. Sounds like you have a 4 wheel drive. Changing your gear ratio just got twice as expensive. If this is something you have just got to do. Put in an OD trans. Lengthing and shorting drive shafts is doable. Moving the engine? Once again what to you want to gain? How much is it going to cost? I have 4:56's with a 4 speed in a K20. It gets around 10mpg. I know even if I change to gear ratio and put in an overdrive I'd only get at most 14mpg. Why? It's heavy and not very aerodynamic. I figure even doing most of the work myself it'd be around $3000. Rather spent that money somewhere else.

[Rotaxx]

I have a '72 3/4 ton with 4.10 gears. I swapped from a TH350 to a TH200-4R.

I haven't checked gas mileage, and I haven't installed my tach yet - so I don't have hard metrics to compare yet. I will say that I have noticed a big difference in RPM on the highway, and also in mileage.

An advantage to using the 200-4R is that it is the same exact length as a TH350, and I was able to use my driveshaft and yoke, and shift linkage. I also used the same crossmember - you just have to slide it back on the frame to the TH400 mount holes. My frame already had the holes from the factory, so it was literally a bolt-on with no modification required.

I went through Monster Transmission in Florida - and was very happy with the experience. It is kinda pricey though. If anyone is interested, I can write it up.

Bill

[68 TT]

Originally Posted by LockDoc

I had a completely different experience with my 2004r and the 3.07. The 3.07 made my transmission overheat in overdrive and puke all of the fluid out of it. Once I switched to the 3.73's no more problems. I got 16mpg driving from central Iowa to Kansas City when I drove the Burb to the All Truck Nationals.

LockDoc

I have had the same experience. Going from 3.42's to 3.73's in my K10 actually improved my gas mileage. The engine didn't have to work as hard to get the truck moving and the RPM's in OD were just right to let it lock up the converter more often and keep it there without having to down shift on slight grades. With the taller gears it was always in & out of OD and converter lock up.

[68 TT]

Originally Posted by ducksfootball82

What modifications are all required? I know the r4 is slightly longer and may require the driveline to be shortened, would the front one have to be lengthened? Could I move the motor mounts forward in the engine compartment to offset the additional length of the r4? What about crossmember and shift linkages? Sorry for all the questions, just want to be thorough before a decision. I was also thinking a monster transmissions r4 for the swap. Would that be a good one or would you recommend someone else?

One of my trucks had the long tail shaft T-350 so it was the same over all length as the 700R4/4L60 so it only required relocating the crossmember to convert it. The 6" tail shaft T-350 is fairly common so you may want to see which one you have. It may save you some money and make the swap much easier to accomplish.

If your engine in in the rear mounting point on the frame you can move forward to make up for the difference in length from a short tail shaft T-350 and save the time and cost of having a driveshaft shortened.

[ducksfootball82]

Thanks for all the replies!

[72MARIO]

What has not be mentioned is the importance of being in the engines power band while in od on the hwy.

You need a very low torque engine for the o/d gear.

Anything less than 3:73 (might get away with 3:42) and 31" tires does not work well. IMO

What size tire are you using ? That make a huge difference in what gear you can use.

A 4:10 gear 33" tire and od would be a great combo for all around driving.

[ducksfootball82]

I have 4:10's and 31's right now but am planning on doing 33's.

[Rotaxx]

When I ordered the 200-4R, they shipped the entire trans and installation kit right to my door. When placed the order, they wanted to know rear end gearing and tire height so they could install the correct speedo drive gear. Something to keep in mind regardless of where you source your transmission. You are a step ahead if you already know it's a 4.10 and your tire size.

I also installed a trans cooler, lockup switch, and trans temperature gauge. Behind a mild 350, the 200-4R runs about 175 degrees (with ambient air temps anywhere between 25-65 degrees). When I flip the switch for lockup, the trans temp drops about 15 degrees over the next 5-6 minutes and will run right around 160 degrees.

Bill

[AzDon]

My burb is 2wd, has 275/60/15's and no interior except a couple of highbacks....I have a lockup switch on the dash, as well as a fourth gear lockup switch and a vacuum operated lockup cutout switch, so I dont operate the lockup going up grades.... I have a glowshift trans temp gauge and don't currently have a trans cooler hooked up.... The trans temp runs right with the coolant temp which is almost always under 180, even during 105+ summer days. I don't tow with it as I have an air-conditioned dually for that!

[AzDon]

Trying to get fuel mileage with a 4by is kinda touchy..... Everyone I know that has tried gearing-up their 4wd for highway fuel mileage ended up with inadequate power for off-roading...

[ducksfootball82]

Thanks everyone, couldn't decide…so for now just rebuilding the TH350 and will run it for a while, then go from there. appreciate all the input

[olblue60@aol.com]

I love mine ! It makes the truck highway friendly. You could also check into finding a good gear venders overdrive. it is installed after your existing trans.