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#1 |
Registered User
Join Date: Feb 2014
Location: Ontario
Posts: 11
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1978 full-time 4x4. Is a 700r4 worth it?
Hello All,
I have a: 1978 Chevy Full-time 4WD NP203 Engine 350 sb Q-Jet th400 Front axle GM Corporate 10 bolt Rear axle is a 14 bolt 4.10 gears 35" tires I want to make any cost-effective changes I can for fuel mileage and DON'T want tbi or part-time 4WD. This points me to a 700r4 transmission. Problem is, based on my calculations I could get good cruising RPMs (60mph @ 2200) from 3.73 gears in my th400. This is a project truck and not drive-able yet, so I have no idea what the MPG is currently. I would be totally happy with 12-15MPG. What would be the preferred transmission and gears? Thanks. |
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#2 |
Registered User
Join Date: Jan 2009
Location: Louisville,Ky
Posts: 5,811
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
To use a 700r4 with your np203 will take a adapter kit and changing out of the input shaft.Or if you don't mind having a np205 in 91 the crewcab had a 4l80 np205 combo but was one year only.The other option is gear vendors offers a overdrive you add on but not sure if it would work with a full time t case or not.
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#3 |
Registered User
Join Date: Feb 2007
Location: Memphis MI
Posts: 1,851
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Do the math.
TH400 to 700R4, with lock up, is about .7 ratio change (since the lockup eliminates the slip of the converter too). Lets say getting 10mpg. 10/.7=14mpg. Let's assume its' your DD and sees 15,000 miles per year. 15,000/10 = 1,5000 gallons of gas, at $3.50, that $5250 a year. 15,000/14 = 1070 gallonss, at $3.50, that $3750 a year in gas. So that's $1500 a year, assuming you can drive it at 60mph and 1540! RPM. Which you can't - you need to be around 1650 rpm at 60mph to tolerate the drive. Really, you need to go to a part time case to get reasonable fuel economy. Here's the key. Aim to make 1600RPM at 55MPH, and find the ratios that work. Think of the 700R4 as a .72 ratio and play with tire size and gearing.
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1987 2 ton ![]() 1982 ![]() Dad's 1981 ![]() Plus a mess o' tractors |
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#4 |
Registered User
Join Date: Jan 2009
Location: Louisville,Ky
Posts: 5,811
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
A easier swap would be the 700r4 np208 or np241.
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#5 |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
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#6 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,357
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
I get 11 mpg with a 350/350/np2o3 with 3.08 gears. I can't give up the 203 because I use it for bad weather. I have a 2wd for good weather and good roads. I think a 700r4 would be a very good swap for these trucks if you do a lot of highway driving. then again if you are doing lots of highway driving you may not need a np203. I would buy a tach first so you know exactly how high it is revving on the highway. It also matters how many miles you drive it per year. If you only use it for a couple thousand miles per year then the actual gas savings won't be so much.
The new Chevy 2wd trucks run 1500 rpm@70mph. I get 16 mpg at that speed. I don't think our old trucks can lug at 1500 smoothly. I think they could be happy at 1800-2000 cruising rpm.
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#7 |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
why cant you give up a 203 for a 208? its just going to give you the ability to select 4wd instead of full time.
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#8 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,357
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
On snow and black ice I like the full time all wheel drive without having to wonder if there is too much grip for 4WD. You can hardly make it go sideways if you try. On marginal roads you don't have to wonder if you should be in 2wd or 4wd; with a 203 you are always in AWD. I already wrecked posi trac vehicles in my younger days on black ice so I love my open axles and AWD over 2wd posi.
FWIW my brother has a new Toyota 4wd truck and that gets 15 mpg on the highway. I don't know how much we can realistically expect out of a square Chev.
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76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10 |
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#9 | |
Registered User
Join Date: Feb 2014
Location: Ontario
Posts: 11
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Quote:
I inputted those numbers into a RPM/Gear ratio/Transmission calculator and it said for 1600RPM @ 55MPH I would need 4.10 rear end gears with a 700r4. Wouldn't that "lug" the motor? I mean, this is a typical speed I would drive to work so it is ideal to make that speed the most efficient. If I put 4.56 gears in, it says that I will run 1750RPMs @ 57MPH. That seems better. What do you think? |
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#10 |
Registered User
Join Date: Feb 2014
Location: Ontario
Posts: 11
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
So a NP208. Do I need to put locking hubs on the front axle or could I just leave it as is? It's a Corporate 10 bolt.
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#11 | |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Quote:
Last edited by nekkidhillbilly; 02-24-2014 at 10:56 PM. |
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#12 | |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Quote:
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#13 |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
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#14 |
Another Day, Another Dollar
Join Date: Nov 2008
Location: North Vancouver, BC
Posts: 1,316
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Part time your 203. You will still have the ability to run part time 4wd just like you have now, you will just have to get out and lock the hubs when you want it.
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#15 |
Another Day, Another Dollar
Join Date: Nov 2008
Location: North Vancouver, BC
Posts: 1,316
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
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-78 K20 Cheyenne 454 long box, 4" lift, 35" MT's, '84 cab, '80 box "its hip to be square" ![]() |
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#16 |
Registered User
Join Date: May 2010
Location: Lewis County, WA
Posts: 1,523
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
The only part of the 203 worth saving is the range box. The rest is a boat anchor.
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'77 K35 - 454, SM465/NP205, 4.56 D60/14bFF |
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#17 |
Another Day, Another Dollar
Join Date: Nov 2008
Location: North Vancouver, BC
Posts: 1,316
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Your right about that but this guy sounds bent on keeping his. Now that I think about it the link for the kit I posted is the wrong kit. There is another kit that doesn't wedge the spiders, but actually replaces the carrier and can still act as a differential when not locked.
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#18 |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
for the money a junk 208 would be what 200 bucks tops and no hurdles to bolt a 700 too.
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#19 |
Registered User
Join Date: May 2010
Location: Lewis County, WA
Posts: 1,523
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
In my neck of the woods NP208s on Craigslist are $50-$100, $150 with both drivelines. Or free if a buddy has one.
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'77 K35 - 454, SM465/NP205, 4.56 D60/14bFF |
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#20 |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
there ya go.
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#21 |
78K & 79C Jimmys
Join Date: Nov 2009
Location: Ottawa Ont CANADA
Posts: 7,901
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
My 78-203 Jimmy is part time with a kit,personaly I wouldn't do it now
the 203 needs to be in 4X4 to lube up,others don't. I have manual hubs,a bad weather day,I leave them locked in,shift in/out of 4 wheel lock as needed But I have been thinking of re-doing it back to full time again In my case we drive it bad weather or storms I find the Jimmy very light and fishtails easy in 4X2 I think the full time again would give me better safer grip but when I got stuck in 4X4 Hi Lock,I undid the front hubs and still drove it home
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John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350 |
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#22 | |
78K & 79C Jimmys
Join Date: Nov 2009
Location: Ottawa Ont CANADA
Posts: 7,901
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Quote:
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John 1978 GMCJimmy4X4-350/203 1979 GMCJimmy4X2-305/350 |
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#23 |
Account Suspended
Join Date: Apr 2009
Location: hazard, ky
Posts: 1,674
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
just lock it in 4wd when you get out in a storm in general if your that afraid of the 2wd. me i just use 4wd when i know its something it wont climb in 2wd.
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#24 |
Registered User
Join Date: Feb 2014
Location: Ontario
Posts: 11
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
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#25 | |
Registered User
Join Date: Feb 2014
Location: Ontario
Posts: 11
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Re: 1978 full-time 4x4. Is a 700r4 worth it?
Quote:
I want that convenience and just wanted to see if there was an economical option to save fuel as well. haha I got the cake and want to eat it too! |
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Tags |
14 bolt, 350 sbc, 700r4, mpg, th400 |
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