1978 full-time 4x4. Is a 700r4 worth it?

[Jankovic]

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.

[andrewmp6]

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.

[D13]

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.

[andrewmp6]

A easier swap would be the 700r4 np208 or np241.

[nekkidhillbilly]

Originally Posted by andrewmp6

A easier swap would be the 700r4 np208 or np241.

what id do shouldnt be hard to find. i had a hummer h3 for about day it had a full time 4wd and overdrive so it was very similar to the old trucks with it. 8-10 mpg was about it.

[cadillac_al]

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.

[nekkidhillbilly]

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.

[cadillac_al]

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.

[Jankovic]

Originally Posted by D13

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.

Too much 'Murican math there! I need KM's and Liters! Just kidding haha. Thanks for the feedback.

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?

[Jankovic]

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.

[nekkidhillbilly]

Originally Posted by cadillac_al

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.

it can be put in in a second. i only go in 4wd when i have to. and ft4wd sucks 99% of the time on dry roads. just my op.

[nekkidhillbilly]

Originally Posted by Jankovic

Too much 'Murican math there! I need KM's and Liters! Just kidding haha. Thanks for the feedback.

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?

i wouldnt think so. my 01 would get in od at 39 mph. was about the same at 55.

[nekkidhillbilly]

Originally Posted by Jankovic

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.

wouldnt have to be a good idea though.

[willett]

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.

[willett]

http://milemarker.com/products/drive...ts-accessories

[Redcap]

The only part of the 203 worth saving is the range box. The rest is a boat anchor.

[willett]

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.

[nekkidhillbilly]

for the money a junk 208 would be what 200 bucks tops and no hurdles to bolt a 700 too.

[Redcap]

Originally Posted by nekkidhillbilly

for the money a junk 208 would be what 200 bucks tops and no hurdles to bolt a 700 too.

In my neck of the woods NP208s on Craigslist are $50-$100, $150 with both drivelines. Or free if a buddy has one.

[nekkidhillbilly]

there ya go.

[motornut]

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

[motornut]

Originally Posted by Jankovic

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.

Welcome,where abouts are you?

[nekkidhillbilly]

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.

[Jankovic]

Originally Posted by motornut

Welcome,where abouts are you?

Thanks for the welcome. I am from Dresden, Ontario. 1hr from Windsor-Detroit border.

[Jankovic]

Originally Posted by willett

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.

I'm not bent on the 203. I just know that I have, on several occasions required 4WD due to snow and mud. It was very convenient to just throw it in 4wd. I wouldn't like getting out and locking the hubs when encountering icy/muddy terrain.

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!