1968 4x4 short step geared to low

[possum1]

I joined this forum about a year ago when I was in the middle of the 68 rebuild. I have had it on the road for about 2 months now (my 16 year olds daily driver). My issue is on the highway at 70 MPH it is cranking 3400 rpm. I did a axle swap during the build and I have approximately a 2.05 rear end. I was going for gas mileage but cannot get the rpm down. Is my problem in the Rockwell transfercase. I can see the shaft does not go straight through as it does on the NP 203 and 205. Pics coming.

[possum1]

Took one and a half years to get her to this point.

[SS Tim]

Stock that truck would have had 3.73 gears. Where did you get a 2.05 rear end from? I cannot recall an application that high for a HO35 axle (12 bolt truck).
What size tires are you running?
Are you sure about the road speed? Did you use a chase car or GPS (not a phone)?
You know rockwells tend to be loud right?

The rockwell t-case is a 1:1 direct ratio and 1.94:1 low ratio.

What have you done for upgrades on the truck?
Do you have the factory service manuals?

[possum1]

I harvested the axles out of a 86 blazer. I have not pulled the cover but, I turn the tire once, and the driveshaft turns slightly past 2 turns. I put 350 TBI heads, Tuned port fuel injection, power steering, electric fans. When I first checked the speedometer, I did it with a gps, When I checked it at 70mph it was with a chase vehicle. I have 31's on it, I know that there is a lot of transmission/transfer case noise, but this thing is screaming going on the highway.

[SS Tim]

Sounds more like you have 4.11 gears. With an open rear end you need to turn the wheel twice to account for the spider gears to get the ratio as a visual at the input yoke.

[possum1]

I have not heard or read that anywhere. Not doubting you though since it would make sense. I do not understand this at all. (I'm a small engine and atv guy.) How can I tell for sure. No tag on rear end. It is not a posi if that makes a difference.

[SS Tim]

Well you can run your numbers/results through a calculator like Summit's here.
http://www.summitracing.com/expertad...tio-calculator

You can take my word for it that the differentiating effect of the side gears (in an open differential) causes the manually turned wheel to rotate around the fixed wheel thus underdriving the pinion gear 50%.

Or there is this quote from a post that sums it up very nicely.
"To check rear end ratio jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. With the tranny in neutral (block the front tires first) rotate the jacked up tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading."

http://67-72chevytrucks.com/vboard/s...09&postcount=8
http://67-72chevytrucks.com/vboard/s...d.php?t=455117

There may also be gear codes stamped in the housings.
Of course the best way is to pull the diff covers and count teeth. No matter how something was "built" or "it should have", there is no guarantee it will still be the same 40 odd years later. Just divide the ring gear (large) tooth count by the pinion gear (small) tooth count.

[possum1]

THANK YOU, this has been driving me nuts! It makes sense now. I will be pulling the cover tomorrow. Since you obviously now what you are talking about, what gear ratio would you stick in there, to obtain the best fuel economy, it will not be used much in 4WD. At least it shouldn't, but my 16 year old does drive it so who knows.

[SS Tim]

Taller gears above the stock 3.73 are only going to give a mileage increase if it is driven to maximize it. That is something 16 year olds typically do not do. It is far easier to lose any gains at every stop than make gains in steady state cruise. A lot will depend on driving habits, routes and conditions. Being as it sounds more like a cruiser/driver than a hauler you should be able to use the 3.07 gears sucessfully. They were stock in Blazers in the time frame with about the same empty weight. Problem is you have late model axles and an as yet unconfirmed 4.11 ratio. Changing both axles is a lot of work and money as would be a ratio change at both ends. Right now this is more food for thought than a hard recommedation.
Also would you confirm the truck has a stock three or four speed manual trans. for me.

[possum1]

The truck has the original 4 speed, which I will need to rebuild also. it has a whine in 4th gear, unless I am getting the noise from the transfer case and it is transfering through the shift lever. I do have 86 blazer axles under her, I did have to change the perches on the rear to get them to match the original springs. So I would think that I am only going to be in it for a gear change.

[geezer#99]

If it were mine I'd leave it alone. If you swap to 3.07's your mileage will drop like a rock. You'll be deeper into the gas pedal just to maintain cruising speed. If the young driver starts having problems getting it rolling in 2nd gear then you'll be replacing the clutch in short order. Starting in 1st can be a pain with the deep compound gear 1st has. And no quick downshift to 1st if needed. If you got room a tire that's 2 inches taller will drop your rpm 400 rpm.
Awesome looking truck BTW!!

[SS Tim]

The Timken/Rockwell T-211 cases are by design noisey. As you noticed it is not a straight through direct drive. My guess is that it is the case and not the trans. but we can explore that later.

If you are thinking of regearing check both axles for matching ratio and carrier type/condition at some point too.

[SS Tim]

Some good points but a 31 to 33" tire change is only about a 190rpm decrease. Maybe something to chase at tire replacement time but not enough to initiate a replacement.
As before give us the ratios you have and if you can a comfortable cruise RPM for your engine.
One point that is not clear is if the engine is a 350 or a stock 307/327.
TPI and TBI heads should give us a broader power band than stock and theoretically can use a higher ratio. Some of the smoggers were running high 2s but were not intended to run 70mph.
There are some variables here like axle type and carrier ranges that would influence a ratio change decision.
Mileage is going to be a tradeoff quick, fast and strong double digit mileage are at odds in this case.

[possum1]

My problem with leaving it as it is, is that we are here in rural Texas. 70 mph is not even the speed limit. 75-80 is the limit all around here. I did not look to see what the comfortable rpm was, but it is around 60 mph. I wired in a temporary tach the time that I checked it at 70. The block is a 307 and it is still has not been bored. If it helps, both front and rear are 10 bolt. I am leaning to sticking 308's in there and see if it fits the way I would like it to run.

[buckskins4ever]

I may be wrong here, but I thought on 4x4's it is required if you do a gear change on the rear...you have to change the front to match otherwise it will have all sorts of drive-ability issues.

[possum1]

I will be changing both front and rear. I may try just the rear to find out if it runs the way I want before doing the front, but I will remove the front drive shaft if that is the case.

[SS Tim]

Originally Posted by possum1

My problem with leaving it as it is, is that we are here in rural Texas. 70 mph is not even the speed limit. 75-80 is the limit all around here. I did not look to see what the comfortable rpm was, but it is around 60 mph. I wired in a temporary tach the time that I checked it at 70. The block is a 307 and it is still has not been bored. If it helps, both front and rear are 10 bolt. I am leaning to sticking 308's in there and see if it fits the way I would like it to run.

If you can try to get a "sweet spot" engine RPM it would help us. This is of course with the actual current gear ratio and tire diameter/rolling radius.

Running 75-80mph is a lot of work for a 307 engine even wih your mods. If you want to run that fast you will need some gear and 3.08 will probably be too high. Without more info I'm going to say something in a 3.50 range is probably the safest.

Taller tires will help some but not at those speeds with a SWB 4x4 and a young driver if it progresses to the point of lift springs.

Originally Posted by buckskins4ever

I may be wrong here, but I thought on 4x4's it is required if you do a gear change on the rear...you have to change the front to match otherwise it will have all sorts of drive-ability issues.

Yes, a good point that may be a little vague in my earlier responses. The front and rear gears need to be very close in a conventional 4x4 drivetrain. Often there is a little lead in the front say a 4.09 with a 4.11 rear but usually very close and under 2%.

[69GWC]

I know my 69 with a 350/4sp with the rockwell and 3:73 gears and 33" tires runs under those rpms doing 70mph IMO its about the perfect gear.
I cant remember for sure but Iam thinking it is turning like 2,800rpm around 65/70mph just been awhile since I had the gps in and the speedo is off 15mph.
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[SS Tim]

If I may, an O/D 5-speed and a late transfer case swap looks pretty good with the presumed 4.11 and even more so if they are actually 4.56 gears. A GM NP260 (80's) 4-speed O/D might be another choice. If we can find one and a matching t-case or adapter.

[possum1]

before realizing that I had 4.10/4.11 instead of a 2 series rear end, I was actually looking at doing a swap to a 5 speed and transfer case. But now I am really hoping that a ratio swap will get me close to what I want. eventually I do want to upgrade to a 350 instead of the 307, but if I spend too much more on this thing right now I might end up divorced!

[SS Tim]

Really get the numbers for us but look around for swap parts. Depending on what you need and can do as far as gear changes they might not be too far apart in price. Trans vs gears. Deals are out there.

[possum1]

I will wire the tach back in this weekend and let you know what I find.

[possum1]

I am working on a trade for 3.08 for my gears right now also, Then I will only be out seals,bearings, shims, etc. they are a matched set from a mid 80's suburban. I have been lucky to have a couple of guys around here that can find damn near anything. We all have our own niche and help each other out.

[72blazerblue]

All I have to say is.....WOWWWWWWWWWW! I love that truck!!!!! That thing is beautiful. Great work, great stance, great color! Very nice. What's the color?

[SS Tim]

Just remember 70 plus is going to suck gas. 3.08s are going to suck off the line compared to 4.11.
So I'd get the numbers together along with a solid baseline mpg. The we can see where everything looks pretty good.