Changing diff gears...

KingArthur · Nov 13, 2017

[font=Georgia,]Hello,

I’ve recently bought this cool old military ambulance. It’s a 1988 G30 Hi-cube with the 3 speed and the 6.2. I’m going to be turning it into a tiny home.

Being that it was an ambulance, i'm pretty sure its got some high gears in the diffs for better acceleration. Since the transmission is a 3-speed, and I would rather have better mileage than towing power, I want to swap out either the tranny or the diff gears so this thing is more of a runner than a hauler.

I would imagine that changing the diff gears would be easier and make way more sense than dropping in a different trans. It doesn’t really bother me at all that it’s a 3speed, I just want the motor to not be roaring near redline while i'm doing 65 on the highway.

There’s a bunch of other things that I could show you all if you’re interested about this van, my plans for it, etc, but at this point my main goal is to make it start getting the sort of mileage I want it to get.

I’ve never put different gears in a diff before. I’ve done my fair share of work on vehicles… rebuilt front suspensions, timing chains, replacing pumps and housings and this and that, but I’ve never cracked open a differential. From what I’m looking at, you can get almost any damn ratio of gears, but you want certain ones depending on your tire size and other factors, etc etc, so I would love for someone to point me in the right direction to begin to educate myself on where I should begin.

At this point I doubt I’ll ever need to tow anything over a ton so towing capacity is totally an afterthought. Once the tiny house is built it will probably have less than half a ton of weight in it so really this thing is going to be light its entire life, as far as I can see at this point, so I want the lowest gears possible that make sense.

Thanks!
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tx2sturgis · Nov 13, 2017

If you go too tall on the ratio, the truck will be even slower than it is now...there's a sweet spot for ratios.

First step is to find out what ratio it has now, and what you want is slightly taller gears, lower numbers.

You might also want to look at the speedometer drive gear, that should be changed as well.

You're going to need an ID on the differential, or axle, some numbers should be printed on it, either stamped on the housing or on a metal tag, and those numbers might help identify the factory ratio. Since it was a military spec, its possible that those numbers are different from standard civilian models.

Or maybe there is a label in the driver door frame or in the glovebox. Again, since it is mil-spec, maybe not.

http://www.sierragear.com/gm-rpo-axle-ratio-identification-codes-3/

http://www.sierragear.com/differential-identification/

highdesertranger · Nov 13, 2017

a few points. like Texas said the first thing you must do is find out what gear you have now and what axle you have. post some pics of the pinion yoke, rear cover, and pop a hub cap off and take a pic. I tell you right off what axle you have. hopefully you have a 14 bolt full float(FF) axle. the easiest axle to work on and the strongest of any light truck axle. you still need a few special tools. I can help you out I know what I speak of, check this out,

https://vanlivingforum.com/Thread-rebuilding-a-GM-14bolt-rear-end

highdesertranger

tx2sturgis · Nov 13, 2017

HDR, some bad ass work there in that thread!

UptownSport · Nov 14, 2017

tx2sturgis said:
First step is to find out what ratio it has now,

Yes, good question, this would seem to be step #1.
Is it actually running too high RPM?
A better solution is getting rid of the slushbox for an overdrive manual-
One tons of that era used Dana 60s or 70s nearly exclusively.
You need a spreader, patience and experience to swap out gears.
There's enough trucks in JY now to find a nifty OD manual, sourcing pedal bracket will be hardest part.

The cause of society's woe were directly caused by automatics.
Remember when you were young and happy?
That's because your old Pinto had a manual, not because you were young

tx2sturgis · Nov 14, 2017

UptownSport said:
The cause of society's woe were directly caused by automatics.

I blame society's problems on air conditioning, but that's for another thread...

highdesertranger · Nov 14, 2017

Uptown that is just plain wrong. GM 3/4 and 1 tons from that era used the GM 14bolt rear axles. the only ones that didn't were the wide dually's, they used a Dana 70 in those. the narrow dually's and single wheel used the 14 bolt. GM used Dana 60 front axles on one ton four wheel drives not the rear. the only reason he needs to ID the axle is because there are 2 different 14 bolt axles, a full floater and a semi floater. you don't need a spreader on a full float 14 bolt. highdesertranger

KingArthur · Nov 14, 2017

Thanks everyone for the input.

I'm going to check today, but right now I believe it has shorter, higher numbers, torquey, whatever nomenclature, in the differential paired with the TH400 3-speed with a 1:1 3rd gear as drive. The van does not have a tach, all I know is that it is howling at 65 and I'd like it to be running at about 2100rpm in drive if I could.

Someone mentioned elsewhere that another viable option would be to, all other things being as predicted right now, to put a 700r4 4 speed into the van instead. I really really like the idea of a 30% OD gear, but what I really want to do is make sure everything makes sense. I want to make sure that I'm not going to spend 2x as much and turn this into a massive project just to have 4 speeds instead of 3. That being said, If I had a 4 speed, it would probably retain a lot of its towing power, while still having highway/economy capability, and I'm assuming I wouldn't have to lengthen the gears in the diff so aggressively. I assume that even if I did get a 700r4 in there, I would want to change out the diff gears anyway to get the ratios right for what I'm trying to accomplish.

Sorry for all the postulation I will get all the ID's and pictures I can today.

bonvanroulez · Nov 14, 2017

To get information about your axle and ratio look for the RPO Code sticker, in a 1988 van it should be in the glovebox. Later vans had it on the passenger door jamb. Sticker looks like this:

Codes starting with F, G, or H have to do with axles. Find an RPO code list, a quick google search turned up this site with axle-specific codes:

http://www.crawlpedia.com/rpo_axle_codes.htm

RPO codes will tell you everything about the build of your vehicle, right down to paint color and finish.

highdesertranger · Nov 14, 2017

I would not put a 700r behind a 6.2. much better would be a 4L80. or a Gear Vender OD on the T400. highdesertranger

UptownSport · Nov 15, 2017

highdesertranger said:
GM 3/4 and 1 tons from that era used the GM 14bolt rear axles.

Yes, recalled the short hubs after posting.

Regardless, a manual swap would likely be about same ballpark and resultant fuel/performance/reliability would be the gift that keeps giving.

The amount of fuel that ended up being burnt to heat the heat exchanger in Americans love of the slushbox couldn't be calculated. Relative a manual, these three speeds were a double digit efficiency penalty.

tx2sturgis · Nov 15, 2017

UptownSport said:
The amount of fuel that ended up being burnt to heat the heat exchanger in Americans love of the slushbox couldn't be calculated. Relative a manual, these three speeds were a double digit efficiency penalty.

It's disappointing to me that NONE of the American pickups (or vans) can be bought new with a manual transmission now, except for a very expensive Dodge work pickup with the diesel, and it is over 50 grand!

In my opinion, any vehicle pulling a trailer over about 1000 pounds should have a 'stick'...but that's just me....

highdesertranger · Nov 15, 2017

unfortunately a large portion of the population don't know how to dive a stick. highdesertranger

bullfrog · Nov 15, 2017

When I bought a standard shift truck the salesman said at least it would be unlikely to be stolen as most crooks don't how to drive one.

Spaceman Spiff · Nov 15, 2017

bullfrog said:
When I bought a standard shift truck the salesman said at least it would be unlikely to be stolen as most crooks don't how to drive one.

Doubly so if it is non-syncro'd

tx2sturgis · Nov 15, 2017

True that.

Still, the choice of a manual transmission should be available in American trucks just like it is with some sports cars and Japanese pickups.

KingArthur · Nov 15, 2017

A manual swap seems to me to be more complicated than it's worth no?

KingArthur · Oct 17, 2018

Resurrecting this thread...

Unfortunately there's no RPO plate. But here's some pictures of the axle and hubs.

MrNoodly · Oct 17, 2018

Minor, somewhat irrelevant point: these are Air Force flight line vans, not ambulances.

MrNoodly · Oct 17, 2018

Looks like a GM 9.5" 14-bolt diff.

<a href="https://imgbb.com/"><img src="https://image.ibb.co/hDLDGL/Screen-Shot-2018-10-17-at-10-06-50-AM.png" alt="Screen-Shot-2018-10-17-at-10-06-50-AM" border="0" /></a>

http://www.differentials.com/gm-9-5-differential-parts/

Changing diff gears...

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