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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: Jeff86inMS on October 11, 2024, 08:47:58 am
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Good morning all,
I have a 1986 stepside custom deluxe C/10 that needs some transmission work done:
I need to chose an option:
Option 1
Rebuild the one we have (TH350)
Option 2
Buy a new one ( same style) that has a longer warranty.
Option 3
Step up to a 4 speed with overdrive (700r4) to get more power out of the new crate 350 engine and provide better gas mileage on both city/highway.
If option 3, I would also need to shorten drive shaft as the transmission is longer, and regear the rear axle to match transmission.
This option would obviously be the most money, but better off in the long run from a drivability and mileage standpoint in my opinion.
Any thoughts? Also what gearing would I need to go to in the rearend? Thanks to any and all the reply/respond! Thanks!!
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What gears are in it now? What size tires are on it? What are the specs on your engine? Do you drive on the highway? What is wrong with the TH350 you have? What happened to the original 700R4?
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In addition, what is your intended use for the truck? Daily driver, work, weekend cruiser, towing toys, ______________?
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Option 1, rebuild the TH350.
The 2 or 3 mpg you will gain with a 700R4 will take YEARS to offset the hundreds or thousand+ $$$ you are talking about in changes. Do the math.
My wife's 2023 Hyundai and my 1995 Geo Metro both get about 45mpg. We drive those when we want to save money going from point A - B.
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Don’t know what rear gearing is in it now. Has the original th350 in it at the moment. Recently changed the 305 to a crate 350. I’d like to get more out of the engine, so my thinking was to change to a different transmission , 4 gears vs 3. My understanding was that if I wanted more low end gearing I could simply change the rear end , keep the th350, but would run too many rpm’s at highway speeds. I was trying to get the best of both worlds, a little more low end for drivability ( the ability to leave a light without some a$$wipe riding my tail) and still be able to drive at highway speeds without turning too many rpm’s. Maybe I just need to stick with the th350 for $$$ sake.
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Don’t know what rear gearing is in it now. Has the original th350 in it at the moment. Recently changed the 305 to a crate 350. I’d like to get more out of the engine, so my thinking was to change to a different transmission , 4 gears vs 3. My understanding was that if I wanted more low end gearing I could simply change the rear end , keep the th350, but would run too many rpm’s at highway speeds. I was trying to get the best of both worlds, a little more low end for drivability ( the ability to leave a light without some a$$wipe riding my tail) and still be able to drive at highway speeds without turning too many rpm’s. Maybe I just need to stick with the th350 for $$$ sake.
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By the 1986 models TH350 was no longer an option. The 3 speed automatic was a TH400. If yours in fact is a TH350 it was changed or it was not made for the US market.
If it was a very early 1986 it may have a TH350C left over from the 1985
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It may be a TH350C.
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I would say it really depends on how you use your truck. If you get on the highway occasionally and get it up to cruise at 60-70 for short periods of time (30-45 minutes or so), then it might not be worth the investment to put in an overdrive transmission. Not knowing your age, tire size, or rear gearing, I would say that you could likely go up to about a 3.55:1 gear ratio and be pretty happy with it. If you can tolerate a little more high revving engine and a little more noise, you could even go up to 3.73:1. I have run trucks and cars on the interstate with 4.10:1 and small tires. The engine really doesn't care that much. It can tolerate cruising at 3,000+ RPMs as long as your oiling and cooling system are in good shape. You however might feel fatigued if you spend too much time on the highway like that. If you do spend a lot of time on the highway at cruise, then I would say it would be worth the investment, not because you will be saving tons of fuel money, but because it will not be so tiring driving a vehicle at high RPM that is also loud.
There are calculators online that can help you determine your rear gearing. If you have a tachometer and know how fast your engine is spinning, you can get pretty close to your rear gearing to know what you have, without pulling the diff cover. I say close, because there is a factor of converter slippage and tire diameter variation that will not make it exact.
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My drive consists at the moment (retire in January) of a 25 mile drive going from 0-55 or 60 mph for 25-30 minutes. This is daily. On weekends I do pull a boat that weighs 1000-1200 pounds depending on gear, and ramp launch said boat. This is 90% of how this truck is used. Occasionally I will get on the interstate to pick the wife an antique or two within a 50 mile radius. I may not even need the 700r4, but the pep off the line and the comfort of an overdrive seemed appealing. But for what I use the truck for, may not need it after all.
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I would think that if you have the money, go for the overdrive transmission. If you are already having to spend money to get transmission work done, it shouldn't be too bad to swap transmissions and likely shorten your driveshaft. You are already a 2wd rig, so no worry about adapting transfer cases, re-tubing front drive shafts, etc. Getting a transmission rebuilt isn't much different than buying a new transmission. I can tell you that lower RPM on the highway with an overdrive is so much less fatiguing at the end of the day. You will spend extra money to bump up your rear gearing, but you already expressed interest in doing so. If you have an open differential, you can also step up to a limited slip diff at the same time. Changing the gearing might require you to change differential carrier series. The thickness of the ring gear increases as you go up in ratio to maintain proper gear mesh.
Anyways...you will hear people say it doesn't pencil out financially, but there are other factors in going to an overdrive that aren't part of the financial equation.
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In this 85, there was a TH350C when I purchased it. 2nd gear was out of it and it went from 1st to 3rd, then locked up after 45ish. I changed it out for a 700R4. The transmissions were real close in length and I reused the drive shaft with no issues. The cross member just needed to be moved is all. I changed the shift indicator and it was like it came that way from the factory. My truck had a 2.73 rear end and 4th really didn't stay in 4th without searching between 3 and 4 till 65 mph which I rarely traveled interstates. I was using it on rural roads mainly. Before jumping in to an overdrive make sure that your rear end is low enough or you will actually go back in mpg, at least I did.
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This is fascinating!
My rig: 78 K10, 350 CID, 4BBL, TH350, NP203, Miles Marker part time kit, MM locking hub.
Few years back I went to a 3:41 from a 3:73. Moved from 31' to 33's, nice stance.
RPM dropped expectedly. 85 mph at 3200 rpm. Not that I drive that fast LOL! I average 65-70 mph, peeps can go around
Elected to forgo front end connectivity, don't use for off road, so she sits idle re 4x. Front drive shaft is removed so she is effectively a 2X.
TH350 is showing signs of giving it up, so considering OD alternatives.
With some guidance from this forum, I am ready to have lift components installed to allow additional clearance of 2", with the plan is to replace '33 with '35 tires.
With that impacting the speed/rpm/torque equation, I'm thinking I should have the original 3:73 rear end installed to assist the lower end take off.
While I'm at it, re-install the front drive shaft to the existing 3:73 front end for a true 4x4.
Question: should I consider a different automatic transmission? If yes, please share your observations/math/TC stall RPM/logic/warnings/passions.
Full disclosure: I don't off road this truck, mainly a DD for the work I do. 25m radius from where I live. When I first got her, I did enjoy moderate Off road adventures. I'm not the young guy anymore. 8)
HOWEVER, I don't see me using this for the long run, so any serious mods I make, I would want to appeal to future buyers. 35" on a short bed will be a statement, so I would like a running gear package to match.
I would like to find a sweet spot, with the the max torque landing in the average speed of 65-75 mph. Something about volumetric efficiency, a vision my auto shop teacher spoke of some 45 years ago. Ponce de Leon?
Hope that's not too much to ask...TIA!
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If you are going to go to the 700R4 with 35"s, definitely 3.73 or 4.11 and swap in an NP208 case while you're at it.
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Thx VZ, my thoughts were going in that direction. Didn't think about a 4.11, that would be peppy off the start, and wind down with OD.
NP 208? I'm not familiar with that one.
The 205 caught my attention years ago. I'll check it out.
I guess there was a reason for full time 4x4 203, maybe a farm truck or something...
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Johnny...with 35" tires and 3.73's and a 0.7:1 OD you would see your RPMs at your 85 MPH drop from 3200 to 2100. With 4.11:1 it would be about 2300.
As far as your 65 MPH cruise, right now it should be about 2450 RPMs. With 35" tires, 3.73:1, and 0.7 OD you would cruise about 1650 RPM. With 4.11:1 it would be about 1800 RPM.
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You guys with numbers at the ready just kill me ;D
Thanks EJ! Very Helpful!
From the time I had my first truck, I was told that your fuel efficiency was best at your max torque RPM point. Any truth to that?
I know there are variables that affect each outcome, just wondering if is true or urban legend?
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Johnny;
Lowest Brake Specific Fuel Consumption (BSFC) occurs at or around peak-torque. That is where the engine is the most efficient with the use of fuel, but usually this does not coincide with the best fuel economy for the total vehicle because of variables outside of the engine like aero drag and rolling drag.
Typically for best fuel efficiency you want to aim for the lowest RPM that keeps the converter locked-up.
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EJ that makes the most sense to date. Thx!
My gut told me that if your RPM is at its highest torque level, then ebbs and flows of torque demand would be in a sweet spot. I mean, your engine is offering the most it can in that range.
Having said that, I guess it's simply a math equation that determines mass/RPM of any given engine spec.
My first truck, "57 Apache 3100 had a 235 CID that developed max torque at 2000 RPM. Pretty slow in the late '70s. OD made quite a difference...
With your calculations in this tread, I will try to determine the max torque of my motor, and try to pin the gearing to stay in that range in my 65 mph cruising.
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Johnny;
I would say 35's and 4.11s, but that means having both front and rear gears re-done, which isn't inexpensive. Couple that with switching over to an overdrive transmission and mating it to your transfer case. It is gonna cost you a pretty penny and it will never pay for itself, but it will make it a much more enjoyable truck to drive and you might be able to recoup some of the investment later down the road, selling it.