Np203 part time conversion?

[iamtherealJayy]

I just got another truck a couple weeks ago and I’m just now getting around to messing with it. It’s a 350/350/np203 truck. It’s the only automatic I own and I got it so the ole lady can drive it some too, but with it being full time 4wd it’s going to get terrible fuel mileage I assume. She’s not gonna wanna fill it up every 100 miles and I don’t really care to either lol. My 87 gets around 13, my 78 was getting 8 probably getting 9-10 currently. I’d love to see 12+ but would be satisfied with 10. So I’m wondering how hard would it be to get a 2wd option in this truck. I don’t need 4x4 at all times since it will stay on pavement and gravel roads the majority of time. I watched a few videos on YouTube and they all showed changing a shaft and gears but every kit I’ve found online shows mile marker hubs and some “nut swap conversion” I believe they called it. I’m just wondering if I’m on the right track. I have a np208 aluminum tcase but I’ve read on here about having to swap driveshafts etc.

 

[75gmck25]

There used to be an NP203 conversion kit that swapped the entire shaft inside the NP203, but now they only sell the 501 kit that uses four pawls that you install inside. It is not as strong, but should still hold up fine. It’s not supposed to be hard to install. You will also need to install a kit to allow the hubs to be unlocked manually.

I have an unconverted NP203, but couldn’t tell you how much it affects gas mileage. I’m using full synthetic in the transfer case, but most of the drag is probably coming from the front axles and hubs.

 

[Bennyt]

The mile marker kit installs easily and quickly. I've only done one but it was easily less than an hour. The 203 isn't the best case but will hold up fine in stock to mild application. But if you start really getting into offroad, you can mate it to a 205 and have a doubler and eliminate the weak part of the 203 and have the ultimate t-case.

 

[iamtherealJayy]

I’m looking for just a reliable rig, I’m not into hardcore off-road by any means my trucks spend majority of the time on pavement, dirt, or gravel. I’m not worried about destroying the tcase I just don’t want to get 2-5 mpg lol I’d like atleast ten. It’s a completely stock 350 as far as I can tell other than cosmetic stuff and dual exhaust

 

[77 K20]

I have had both a converted 203 and a full time 203. I didn't see that much difference in fuel mileage. According to my mechanic maybe you'd see 1 to possibly 2 mpg difference. It would take a while to recover cost of the parts and install for a part time conversion.

I had a '77 K5 Blazer. It could get 14 mpg on the highway with stock sized tires. I believe it had a 3.42 gear ratio. Trucks usually seem to have a bit lower gears.

 

[SirRobyn0]

I have had both a converted 203 and a full time 203. I didn't see that much difference in fuel mileage. According to my mechanic maybe you'd see 1 to possibly 2 mpg difference. It would take a while to recover cost of the parts and install for a part time conversion.

I had a '77 K5 Blazer. It could get 14 mpg on the highway with stock sized tires. I believe it had a 3.42 gear ratio. Trucks usually seem to have a bit lower gears.

This. The thing is you have to crack the transer case, and install the locking hubs, so you'll probably have about day or so into the job, and several hundred dollars (not sure on the kit cost anymore) and you might get 1MPG.

I'm in a similar boat, but not with a square with my 96 Grand Cherokee. It's got a full time transfer case. There is no kit for those you have to swap cases, in the end I decided that if for some reason my wife needed to drive it in the snow, just getting in a driving without her having to think about the transfer case was the best idea.

So my vote would be that you guys drive the truck around work out the other bugs if there are any and see what you actually see for mileage. You might just decide that the full time transfer case isn't so bad after all.

Doing stuff like full tune up, timing set carb adjust ect, if needed will likely yield a better improvement in mileage than the T-case conversion.

 

[iamtherealJayy]

It for sure needs a tune up, for a supposedly fresh rebuilt qjet I’m having a lot of problems with stalling and bogging under load. I assumed the full time would make me get 4-6ish mpg and I’d really like atleast ten lol

 

[Big Ray]

mine is full time with locking hubs. I averaged 10-11 on my trip. Included mountains.
That's what I call indicated mileage. With the tires on it I'm off 12 - 14 per cent on the speedo.
So the actual mileage is either higher or lower lol.
Its confusing lol.
Mine has a 400 in it. Needs de-smogged and warmed up.

 

[ali_c20]

I did a 203/205 swap and gas mileage did not improve significant.
Just saw this is an older thread so what was the outcome?

 

[Big Ray]

I have had both a converted 203 and a full time 203. I didn't see that much difference in fuel mileage. According to my mechanic maybe you'd see 1 to possibly 2 mpg difference. It would take a while to recover cost of the parts and install for a part time conversion.

I had a '77 K5 Blazer. It could get 14 mpg on the highway with stock sized tires. I believe it had a 3.42 gear ratio. Trucks usually seem to have a bit lower gears.

It's odd how they put lower gears in the older nonoverdrive rigs and higher gears in the overdrive rigs.

 

[iamtherealJayy]

@ali_c20 i haven’t done anything yet. I’m still in debate. I’ve just heard the 203 gets poor fuel mileage, wears parts faster, eats front tires, etc. the truck is being built to be a cruiser so I’d like a 2H option for highway driving

 

[75gmck25]

An unconverted 203 is very durable, but the front end parts are always turning and that eats into gas mileage and increases wear. The internal chain may stretch and get noisy, but it’s very uncommon to have it break. They are very popular for plow trucks because in the non-Loc positions they are AWD and you get good traction without driveline binding.

It’s easy and cheap to convert one to part time with of the kits that have four pawls and some shims. However, converted 203s don’t run the internal oil pump in 2WD, and you have to run it in 4WD about once a month to maintain good lubrication. The old full internal shaft conversion is no longer available.

 

[74Blaze]

My NP203 which had a conversion kit in it long ago installed had a catastrophic failure last spring. I replaced it with a un-converted used NP203. I'm happy with the un-converted so far. I haven't noticed much difference in fuel mileage. If you convert yours, you will need lock-out hubs on the front axle as well.

 

[Octane]

I get 11.5 to 12mpg w/ np203.pretty easily. 77 k10

 

[scrap--metal]

It's odd how they put lower gears in the older nonoverdrive rigs and higher gears in the overdrive rigs.

Very odd indeed... My '84 had 2.73's with a 700R4, and that was the combo from the factory.