Changing gear ratios: opinions/suggestions wanted!!

Had similar situation. 79 GMC c1500, crate 350 (installed 2004) w/TH350, stock 3.73 rear ratio. Man, tac'ing 3,000-3,200 doing 55-60. That crate motor eventually wore out. So, last year, installed another (this time Jaspers, so far running great, about a year) 350 motor. I'm like.. i need to either swap rear end gears or get different transmission. thing is I like the "pep" in city driving. (I mean, I'm not uhm doing any burn-outs or drifting or anything ;-) as I have a shell, it's a lot (of weight) to haul around, and the low gears seemed to help. So, I decided to go with a non-electronic (manual switch, like I'm launching a missile), 700r4 transmission. Four gears, but I don't use the fourth gear, unless I'm popping on the FWY or long drives. Around town, (with the switch not engaged), it performs just like my old TH350. But man, the fourth gear is great. I get up to like 45, on FWY, take my foot off the gas for a second, flip the switch (I mean launch a missle) and Beee-BAAAAUWW, drops into a very nice Overdrive. 65-70 at like 1,900 RPM's. amazing. As it should be. The motor will not scream at me, and hopefully both transmission and engine will last a bit longer.

So, with going from TH350 to 700r4, couple things happened. They (shop) said OEM driveline was fine. (even tho the 700 is longer (inch or so) than the TH350 housing. I think they just flopped the OEM crossover, I believe holes were also there for this at the factory. I was like, that's not right. (same driveline) they assured me it was. Well did run fine. but.. after like 3 weeks, when i parked on a grade, I came out to a huge puddle of ATF under it. Turns out, the stock yoke for the TH350 in 1979, at least mine, had a little breather hole in it. not sure exactly what for.. - uhm breathing? ;-) . The OEM TH350 never leaked (at least from the yoke) so, I looked around and turns out the 700r4 yoke does not need a breather hole. So we ordered a new yoke, and while that was going on, did have local driveline shop take a measurement and cut it to the proper length. (Also we replaced all three u-joints while we were at it and balanced it. No more leaks, and I'm able to drive on the FWY now. (I've always wondered what it would be like to be in those far left lanes).

* UPDATE. Ooops... what tires.
I've always run them pretty much stock, on steel rims.
The tires on it are "General Grabber HTS 60"
255/70 R 15 108 S M&S

chris3353 said:

Before swapping the gears,, look into a 4l80,, a lot of donors available. I use the MSD atomic controller and it has been flawless for 10 years now.

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Hey Chris, that's good to hear about the MSD controller. I have a 69 C10 with a 350/TH350, but even with 3.08 gears and 29" tires, I find myself looking for a taller gear at 70mph. A 4L60E might be reliable, but an 80 would be bulletproof. I dealt with several issues with an 85 700R4 in a friend's hot rod. No issues with TV cable and converter lockup, but all kinds of other stuff. Turns out it was a cheap rebuild by a trans shop, and it must have had a lot of miles on it before the basic rebuild.

MikeB said:

Hey Chris, that's good to hear about the MSD controller. I have a 69 C10 with a 350/TH350, but even with 3.08 gears and 29" tires, I find myself looking for a taller gear at 70mph. A 4L60E might be reliable, but an 80 would be bulletproof. I dealt with several issues with an 85 700R4 in a friend's hot rod. No issues with TV cable and converter lockup, but all kinds of other stuff. Turns out it was a cheap rebuild by a trans shop, and it must have had a lot of miles on it before the basic rebuild.

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You have a roller cam or flat tappet? Use a gear ratio calculator,use some common sense,like are you at 70 plus most of the time,or just occasionally,etc. I fully believe part of the flat cam problem is too low of rpm on flat tappet cams,they are splash lubricated. Too low rpm I don't think the cams get enough oil.

Ricko1966 said:

You have a roller cam or flat tappet? Use a gear ratio calculator,use some common sense,like are you at 70 plus most of the time,or just occasionally,etc. I fully believe part of the flat cam problem is too low of rpm on flat tappet cams,they are splash lubricated. Too low rpm I don't think the cams get enough oil.

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Flat tappet cam, yes. But it was installed when the engine was built 20 years ago, broken-in properly, and zero problems since. Most of those cam/lifter failures are right after start-up, or these days due to improperly ground cams and lifter faces. But yes, I don't spend a lot of time on freeways, with high revs being one of the reasons.

BigT said:

My 78 K20 4x4 has factory 4:56 gears in it and a TH400 transmission. These are great when off-road or pulling something, but poor on the highway. I grew up with a 75 half ton 4x4 with 3:73 gears and a 4 speed manual, which was very versatile for both off-road and highway, but that was partly due to the 4 speed manual transmission. I'd like to switch out my low gears for something more versatile, but I can't decide on 3:73s or 4:10s, as I don't have an experience with either ratio in combination with the TH400, since I just bought the truck a few weeks ago. I'm leaning toward 4:10s, as I used to have a 74 F150 with something close to that combo, and I liked it. But, the 3:73 is the more common one and gets better mileage, etc. We've touched on this in another thread, but I'd really like some more thoughts from other folks who have experience with either. Thanks!

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If you really use it as a 4X4, I would go with 4:10, but otherwise 3:73 is better with an automatic. If you have a 454, then I would go with 3:73 if you really use it, otherwise something around 3:50 would be better. Automatics will allow you to pull taller gears than a standard trans will.

J. B.

Well, to complicate things - you could swap in a Tremec 6 speed. Just sayin. The 700R4 was the transition from the TH days. It has an overdrive. A good builder can beef up a few parts that made it fragile. Note: It has a TV cable. Super important to get that set right. The successor was the XL80 line. 4L, 6L, 8L ... but I'm assuming you want to avoid installing a computer. Changing gear ratios via the diff is a simple operation in those old units. If you have a dial indicator, it is DIY.

BigT said:

My 78 K20 4x4 has factory 4:56 gears in it and a TH400 transmission. These are great when off-road or pulling something, but poor on the highway. I grew up with a 75 half ton 4x4 with 3:73 gears and a 4 speed manual, which was very versatile for both off-road and highway, but that was partly due to the 4 speed manual transmission. I'd like to switch out my low gears for something more versatile, but I can't decide on 3:73s or 4:10s, as I don't have an experience with either ratio in combination with the TH400, since I just bought the truck a few weeks ago. I'm leaning toward 4:10s, as I used to have a 74 F150 with something close to that combo, and I liked it. But, the 3:73 is the more common one and gets better mileage, etc. We've touched on this in another thread, but I'd really like some more thoughts from other folks who have experience with either. Thanks!

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The 700r4 has a I believe 28 spline output shaft, while the 4l80e or 400 have a 30? thats a consideration. T, you gotta get up here, I have 700r4, 4l80e's, and all the stuff that goes into those, like the 205 big bearing input for the 400, (like yours.) I am swapping the C20 dezzle this winter to a 4l80e. It goes down the road at 65 just dandy, but I drive on occasion in Idaho, Nevada, Utah where the speed limit is 80, kinda scary doing 65 when everyone else is doing 85. You spend more time looking in the rearview mirror then ahead, unnerving. Also stated many times 300-3500 rpm is doable, but it puts me on edge more, and it is louder in the cab as well. I bought several 4l80e tranny's for around 500 bucks, low miles, (40000'S). Money's tight, and I like the 4:10 in both the trucks, as I use them as well, er, trucks. Manual trans you need to source all the pedals and linkage, while the auto is cheaper and easier. Come on up and drink a beer one of these days!! Oh yeah, gotta cut your driveline as well, both are longer by a few inches, drill a few holes on trans cross member.

Craig Nedrow said:

The 700r4 has a I believe 28 spline output shaft, while the 4l80e or 400 have a 30? thats a consideration. T, you gotta get up here, I have 700r4, 4l80e's, and all the stuff that goes into those, like the 205 big bearing input for the 400, (like yours.) I am swapping the C20 dezzle this winter to a 4l80e. It goes down the road at 65 just dandy, but I drive on occasion in Idaho, Nevada, Utah where the speed limit is 80, kinda scary doing 65 when everyone else is doing 85. You spend more time looking in the rearview mirror then ahead, unnerving. Also stated many times 300-3500 rpm is doable, but it puts me on edge more, and it is louder in the cab as well. I bought several 4l80e tranny's for around 500 bucks, low miles, (40000'S). Money's tight, and I like the 4:10 in both the trucks, as I use them as well, er, trucks. Manual trans you need to source all the pedals and linkage, while the auto is cheaper and easier. Come on up and drink a beer one of these days!! Oh yeah, gotta cut your driveline as well, both are longer by a few inches, drill a few holes on trans cross member.

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Hell ya! I'd like to get up there during daylight hours, which makes it tough during the week, and I was sick last weekend. Maybe this weekend? I feel a little odd just popping in without setting a time first though.

I wouldn't put a 700 in a 3/4 ton truck, that's a step backwards.

It is if you want to do 20 series things.

This thread is a rabbit hole. lol. Having done about 95% of what everyone has suggested I can tell you what I settled on. But as was stated, different strokes for different folks. I will say this; it's like that song about bones, they are all connected. Gaining OD is great until you realize your engine is now underpowered. Or that doing 75 in a square has about as much wind noise as the front seat of the space shuttle. Find yourself in bumper to bumper traffic on the interstate and you realize the brakes suck, shifting sucks, the radio sucks, the bench seat sucks, the exhaust smell sucks, the gas mileage sucks....and down we go.

SquareRoot said:

This thread is a rabbit hole. lol. Having done about 95% of what everyone has suggested I can tell you what I settled on. But as was stated, different strokes for different folks. I will say this; it's like that song about bones, they are all connected. Gaining OD is great until you realize your engine is now underpowered. Or that doing 75 in a square has about as much wind noise as the front seat of the space shuttle. Find yourself in bumper to bumper traffic on the interstate and you realize the brakes suck, shifting sucks, the radio sucks, the bench seat sucks, the exhaust smell sucks, the gas mileage sucks....and down we go.

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Right on!!

SquareRoot said:

This thread is a rabbit hole. lol. Having done about 95% of what everyone has suggested I can tell you what I settled on. But as was stated, different strokes for different folks. I will say this; it's like that song about bones, they are all connected. Gaining OD is great until you realize your engine is now underpowered. Or that doing 75 in a square has about as much wind noise as the front seat of the space shuttle. Find yourself in bumper to bumper traffic on the interstate and you realize the brakes suck, shifting sucks, the radio sucks, the bench seat sucks, the exhaust smell sucks, the gas mileage sucks....and down we go.

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That's why I suggested just renting a truck for those long trips he does 1 to 3 times a year and just leave his truck alone to enjoy as GM intended. But, what do I know

Based on all of your really useful info, opinions, and experience (thanks, that's what I wanted), here's where I've landed for now. My original plan was to do nothing for at least 6 months, and I'm staying with that. I definitely like it the way it is, so driving it with 4:56s isn't breaking my heart. I'll continue to do research, and should have a better idea in 6 months of the costs involved, and how I'm gonna use the truck the most, which will speak to the future gear plans. It's an awesome truck just like it is, and there are other things I need to tackle as well, so it may just get put off for a lot longer, if I ever do it at all.

Hell, I still need to re-educate myself on how to use a carburetor again, apparently. I've been driving EFIs for so long I that I keep flooding the old Q-jet when it's already warmed up!! Like a dang newbie again!! Geez, so embarrassing.

Thanks again!!

Radiohead said:

It is if you want to do 20 series things.

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Twenty-five years ago if you would have told me anyone was buying a 3/4 ton truck just to commute with I would have laughed in your face. Nowadays its commonplace. Hell I wonder why Denali/Platinum/Laramie trucks even come with hitches..