K5 Blazer GEARS?

[93BLAZER]

Well I think I'm ready to drop some coin on some new gears for my 87 K5 Blazer. I just have a few questions...

I have narrowed it down to 4:56 or 4:88 gears.
Does any one have any input one which one? I'd still like to keep the fuel economy the best I can and keep the RPM's down.

I have a stock 350 w/ 700r trans.

Also, more importantly... Changing the gears, is that a pretty simple operation? Is it pretty much plug and play so to speak? Is this something I could do alone?

Gracias

 

[alexrex20]

do you really need to regear your K5? you're "only" running 35s so why the jump to regear? 4.56 and 4.88 are a HUGE jump from what you have now - probably 3.42. if you're worried about fuel economy, i'd recommend only going to 4.10s; even 3.73s would make a marked improvement in performance.

if you have to ask how to do a r/p swap, then you probably shouldn't do it yourself. you'll need basic hand tools, but an impact would be preferable. a large table vise will come in real handy, and you'll need a dial indicator and a torque wrench.

your chevy uses shims in the diff to set the backlash (the meshing between the ring and pinion), and is a PITA to set (compared to ford/nissan/etc. which uses threaded adjustment collars).

the backlash is a crucial measurement that you must get perfect. additionally, you need to get the ring gear to contact the pinion at the perfect spot.

if you read up on the process, make sure you have the tools and equipment, and take your time, you can do it. it's just labor intensive and there is no margin for error.

 

[Grim Reaper]

I ran 4.10's with 35's on my 75 Jimmy. It was a good combo but I lacked OD. I would go with the 4.56 gears.

Now here is the money part as I have done the half ton way. I bought a used set of axles at a excellent price then upgraded to used alloy shafts and ended up havign a LOT of money into my half ton gear and I was getting hella deals.

Gear change CAN be done by a novice but it is not a quick and easy job. There is a lot of measuring to be done.
If you need bearings you are looking about $600+ in parts for gears and master rebuild kits. If you cannot do the labor you are going to drop $1500+ at a shop including the gears.
That puts you in the price range where you could buy a set of one ton axles from a Military K30 that are already geared 4.56 with a Detroit in the back.
The Front D60 is going fro around $1200 on the used market. The 14 around $200. May need Yokes or a special Ujoint to get the drive shafts to work but it is otherwise a bolt up. If you get lucky you might be able to score the drive shafts. The front would be a bolt in the rear would be a mater of getting the length changed.
The ugly part is you are going to have to jump up to a 16 inch rim and get a new set of tires but you may be able to sell your 15 inch stuff to cover a good bit of that.

 

[Rot Box]

Changing your gears should be a big improvement and with your larger tire size and factory gear ratio I would say the swap is a must. You are right 4.56 or 4.88's are going to be your best option I would say 4.10's minimum. If this were my truck I would go with 4.88's because I typically drive a little slower on the freeway and am normally loaded down pretty heavy. That being said it is not an easy swap. I don't like doing them without a lot of specialty tools (large press and pullers, extra set of "dummy" bearings and dial indicators/calipers.

I like to match my engines peak torque RPM with the speed that I typically travel while driving down the highway. Scroll down a bit to find the right formula in this link http://www.rocky-road.com/calculator.html Hope this helps.

Andrew

 

[jp0863]

K-5's are heavy and if you load it up with offroad gear, i would definately go with the 4.88's. 4.88's will only be about 180-200 rpm's over the 4.56's so you're not killing your mileage. The 4.88's will make it feel a little peppier over the 4.56's too. As for the labor, I would find a good local shop and spend the money to have it done right. Where are you located? I have regeared several vehicles in the past and you cant go wrong with going a ratio lower than you first think. Just my $.02

 

[ntsqd]

My '91 Sub has 3.73's and 285/75R16's. Doing the math I find that to put the engine speed to road speed ratio back to stock that tire size wants 4.11's. Even though the 700's OD ratio isn't super tall I have found that manually keeping it out of OD until indicated 60 mph results in better mileage than letting it shift at it's programmed shift point. Since the only thing that I've ever towed with it weighs about 1200 lbs. the 3.73's are fine for my use.

My advice would be to look at what the stock rpms at various speeds would be with the stock tire size. Then find the ratio that puts the engine back to those rpms at those speeds with the tire size of choice. If you crawl a LOT or tow a LOT then consider one step lower.

I'm not normally one to advise going to heavier axles when the use doesn't require them, but I'll echo Grim's thinking. Even if you just do the 8 lug upgrade to the existing front axle (Much better brakes!) I would seriously suggest looking into dumping that c-clipped rear axle and going to a 14bff rear.

 

[charlieaarons]

I second the advice about the military 1 ton axles. The 14 bolt is "bulletproof", very easy to work on; once you get used to a Detroit Locker you will never look back. You always have the option of an ARB in the front.
There's little difference between 4.56 and 4.88 for crawling, but a noticeable difference at highway speeds. It all depends on precise tire size; the 700R4 has a tall 4th gear.
If you want slower crawling there's always 3:1 LoMax gears for the 205, or a ORD NP203 doubler or a Klune-V reduction box.
In other words, pick your axle gears for optimum highway efficiency. You can always get a lower low range.

Charlie

 

[85CUCV]

I'm running 4.10's on 33" tires behind a TH400/208 and 6.2L diesel in my 86CUCV. I like it combo in the diesel. I'm running a disk braked, 6 lug, 14 bolt semi float from a 1994 3/4 ton. It is a big strength improvement over the stock 10 bolt rear.

Although if I was starting again I'd find a military Dana 60/14ff combo. I got alot of money wrapped up in re-gearing and locking a semi-float rear and re-gearing and tru-trac installation in a 10bolt front with little front axle strength gain.

A 10 bolt rear is not worth re-gearing expense.

 

[KLAKEBRONCO]

4.56 for sure.
With a 4.10 or higher gear you are running at lower RPM, but anything below 70mph is going to start lugging the engine and causing you to either use more fuel, or shift in and out of overdrive.

If it were somthing that doesn't drive long distances I might even go with 4.88's. You will run a little higher RPM on the highway, and 'might' use more fuel, but city or possibly even rural highway mileage will be better with the lower gear.

 

[TimW]

my old 87 blazer i had 4:10's with 36's and it was still a little slug-est The Blazer now has 4:56's init is a great match with the 36's for an all around vehicle. Now the new owner is going to put 5:12's init as it is on 44's now.
350 eng., 700r trans, 14 bolt rear, dana 60 front.

 

[hidesertwheelin]

I have 4:88's with 35" tires on my Suburban. I am very happy with it.