O/T Chev truck rear ends again

old · Dec 7, 2009

Well I hauled in a Suburban that has a 12-40 tooth ratio which is by my figures a 3:42 so that will not work out well. The rear end I have in it right now is a 13-41 which figures out to 3:15. Just got another rear end that is a 14-43 which comes out to 3:07 so it the 14-43 close enough to get me by or do I need to find a 13-41??
Thanks

JDemaris · Dec 7, 2009

I never heard of the 3.15 ratio. This is a 10 bolt, 1/2 ton rear with 8.5" ring gear, correct? I suspect you've got the GU5 rear which GM calls a 3.23 ratio.

What year is your Suburban?

I you could do fine with a 3.07 or 3.08 by playing with your tire sizes a little bit, and it will be fine. If anything, you want the front drive going just a hair faster then the rear, and never the other way around.

keh · Dec 7, 2009

Old, I don't claim to be much of a mechanic and certainly not a 4wd mechanic, but I don't see how different ratios on the front and rear end would work out at all.

A thought. Does it work if there are different sized tires on the front and back with the ratios the same? Looks like that would have the same effect.

Another thought. My only experience with 4wd is in daughters 97 Tahoe. Instructions for that one are to shift into 4wd drive only when stopped. Use of 4wd on dry pavement is discouraged. Looks to me like different axle ratios would cause the same possible problems even in slicker circumstances and be disastrous on dry pavement.

Well, that's just opinion on my part and some one with lots of 4wd experience can give better advice.

KEH

old · Dec 7, 2009

The 1980 Chev 4X4 truck that I am fixing has the 13-41 in the rear right now, not sure what it really up front but know on most the front is a tad bit higher then the rears to help make it corner better. The one from s suburban is the 12-40 which comes out to be 3:42 which I know is to far off to work. The one I got today is the 14-43 which is a 12 bolt as if the one in the truck now. Also the one from the sub. is a 10 bolt not a 12 bolt. Oh by the way I only use the 4 wheel drive when it is slick out or I'm pulling something out and I start spinning when I do like the time I pulled an OC-3 crawler out on slick wet grass

JDemaris · Dec 7, 2009

Of course it can work with different gears. They do not determine the overall ratio. Same axle ratios need same diameter tires. Unequal ratios need unequal diameter tires.

If he has a 3.23 ratio in the rear, and he puts a 3.07 ratio in the front . . . then all he has to do is have tires that are 1 1/2" smaller diameter in front to make all the wheels turn the same speed.

3.23 with 29" tires turns the same speed as 3.15 with 28.5" tires and that turns the same as 3.07 with 27.5" tires. All available in standard tire sizes.

Technically, 4WDs are supposed to be engineer with the front drive turning just slightly faster then the back. It makes the rig go straigher and handle better with the front pulling more then the back is pushing.

JDemaris · Dec 7, 2009

Let's assume that rear actually IS a 3.15 ratio, although I've never heard of one.

If you use that rear with standard 235/75-15" tires . . .

You can then put the 3.07 in front and use 225/75-15" tires and all the wheels will turn the same speed. Problem solved.

The 235s are 28.9" diameter
The 225s are 28.1" diameter

npuller · Dec 7, 2009

HI old if you use the 4wd just when its wet
the 3.07 will work fine.Just put it in 2wd
before you get on the pavment. you may have to rock the truck back an forth to get back in
2wd if your on dry ground.My x boss had one
with 3.42 ft 4.10 rear.worked fine wet.

TominKy · Dec 7, 2009

Old: I had a 1980 suburban with 4-wheel drive that could not be driven on dry pavement, so obviously the gear ratios were different fore and aft and allowed for some slip on slick surfaces. That being said I have no idea what the ratios were front to back, but from that experience, I would say that if driven in four wheel only on slick surfaces the close ratios will be fine.

Tom

Tim in OR · Dec 7, 2009

Old, I had a 75 chevy LUV that I combined with a 79 4x4 LUV. The rear end on the 79 had been bent in the roll over, so I shoved the 75 rear end in it. If memory serves me, the 75 had 4:35 gears, and the 79 had 4:10's. I figured the rolling distance of the rear tire per revolution. Then I figured how far per revolution of the driveline. After that it was just a matter of figuring what the circumfrence of the front tires needed to be. It wasn't perfect, but was good enough for slick surfaces. I ended up with 15" tires on the rear and 14's on the front. People say you can't do that, but I did with a little calculator work.
good luck.
Tim in OR

old · Dec 7, 2009

Well as of right now I have no idea what the fronts really are but would figure should be close to the 13-41 of the rears but who knows on something as old as a 1980. I run 9:50-30-15s front and rear and plan to keep them that way. Reason is I have warranty's on them so as to cost me little or nothing to replace them if they go bad. I jsut know if winter does kick in I might need to have 4 wheel drive and if I'm thinking right the 14-43 will be close enough to get by if I need it. But I am beginning to understand that Chev did not put one standard rear/front combo in there trucks

Anonymous-0 · Dec 7, 2009

IIRC, some old jeeps had a slightly FASTER ratio in the front axle, so it would pull and keep the vehicle going straight..

MarkB_MI · Dec 8, 2009

As I've said before, new parts to rebuild your differential are reasonably priced. Maybe you don't value your own time, but you're sure going to a lot of aggravation to save a few hundred bucks.

trucker40 · Dec 8, 2009

The front rear end is probably one size higher anyway.If I remember right you had some cross gears that were broke.If the ring and pinion are good why not take the cross gears out of one of them and fix yours?Also the higher one would probably work anyway as long as you use the 4 wheel drive on dirt only.It probably wont steer good on pavement because the back would be pushing the front if it is a little higher in the back.It might actually be the same as the front and then It probably would work on pavement.If it doesnt work in 4 wheel drive at least you can drive it in 2 wheel drive until you get the right one.

old · Dec 8, 2009

New parts are $350 plus and this rear end was a simple swap one for the other and since the 14-43 is a lot closer then the 12-40 is I figure I can get by with it better

old · Dec 8, 2009

If it was the spider gears etc. I would take them from the other rear end but it is a bad pinion gear so that is why I'm changing out the whole rear end. Also since I'll have it apart I am beefing up the rear springs since I have both the set that is on the truck and the set off the suburban. Plus one spring on my truck is broke so I need to take care of that any how. This truck is pretty much only used to haul tractors and hay and when I need to get out in snow or ice

440roadrunner · Dec 8, 2009

(quoted from post at 20:40:22 12/07/09) Old: I had a 1980 suburban with 4-wheel drive that could not be driven on dry pavement, so obviously the gear ratios were different fore and aft and allowed for some slip on slick surfaces. That being said I have no idea what the ratios were front to back, but from that experience, I would say that if driven in four wheel only on slick surfaces the close ratios will be fine.

Tom

Tom you can't do this on many 4x4's. There's enough wear on tires, or even difference between tires, that you cannot and should not run conventional 4x4's on dry pavement.

My old Toy Landcruiser had identical front/ rear--4:11

Many US 4x's have slightly mismatched ratios. For example, nearly all (especially older) 4x4's used Spicer front axles and not necessarily Spicer rear. This meant that your Spicer 3:54 or 4:10 front might not match your "Corporate" 3:55 or 4:11 rear, thus you were "off" right out of the chute.

Except for "full time" 4x4's no conventional transfer case was ever meant to run on pavement, wet or dry.

I've never heard of some of the ratios being discussed here. I would suggest you recount the teeth. By the way it's REALLY inconvienent to refer to an axle by tooth count, since no one else does.

old · Dec 8, 2009

That is why I did it like this 12-40 is the 3:42 and the 13-41 is a 3:15 and the 14-43 is a 3:07 and that is how I think I put it in the first place. I also have a 4:11 rear end but don't remember the tooth count and since none of them hag tags I had to use the tooth count to figure them out

JDemaris · Dec 8, 2009

They became fairly standardized after 1982. Just about all 1/2 ton 4WDs are going to have 3.08s or 3.42s.

As to running 9.50s in the winter? I'd never do it. They can make a death trap out of a pickup on ice and snow. For winter driving the smaller and narrower tires work better. A 225/75-15 or 235/75-15 will run circles around those fatter tires in bad weather. Stop a heck of a lot better too. I've got pile sof 9.50s and 10.50s laying around, some almost new. They're awful in the winter and rob fuel mileage in the summer. All show and no other truly useful purpose except maybe for mud and sand bogging.

You said " 9:50-30-15", so I assume you mean a 9.5" wide tire by 30" diameter. The standard 235/75-15" tires are 29" diameter and 6.5" to 7" wide and much better in bad weather.

My crappy little Subaru 4WD wagon with 185/70-13" tires will do better then any of my big trucks as long as it gets ground clearance over the snow.

old · Dec 8, 2009

What I have is the 9:50-30-15s all the way around and have for years. I have always had good luck getting around with them but then the truck is not a light weight truck as in it is around 7000lbs empty so it is a bit on the heavy side. Plus in this area we do not get all that much snow but we do get ice and as I said I drive any place I need to with this set up

JDemaris · Dec 8, 2009

Yes, finding 15" tires that can handle 7000 lbs. is not easy. I've got a 1/2 ton diesel Blazer that weighs 7200 lbs. It's a camper rig and I did not want anything larger then the 235/75-15". Finding high-load capacity 6 or 8 ply tires was not easy, but I found out that Cooper makes them.

I wasn't picking on your tires. If you like them, that's all that's counts. For me, I can't stand them. They ruin handling if you put them on a short wheel base rig like a Blazer, and are awful in the winter conditions we get here.

old · Dec 8, 2009

When I got the truck years ago it had 12:50-33-15s on it and boy was it squarlly so when I got the chance I put on the 9:50s and they work well. I do also like to haul loads that have been on the scale at 14,000 so I make the old truck work hard at times

O/T Chev truck rear ends again

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