M1009 rear end replacement

[BigDuck]

I had some chips and one broken tooth on the spider gears so I started looking for a replacement. My uncle had a chassie and drive train (no body) that he is gonna put 1 tons under so he sold me the front and rear end for 250$. He said it was a 3.08 but I swaped the rear end first (because of the chips and broken tooth and I'll do the front next month) and now my speed-o is 15mph off..... not a 3.08!! any ideas on what ratio im dealing with??? Its 2x4 only until I swap the front end. but my fuel economy has inceased.

 

[niferous]

15 mph slower or faster?

 

[QuickSilver]

I would just do the ol jack it up and mark the driveline mark the tire trick. See how many turns the drive line takes to turn the tire and there ya go.

 

[M1009 Fiend]

How, hard/ easy was the rear end swap? I might try and do one this weekend if i can.

 

[The YETI]

Pull the cover off the rear axle and devide the ring gear teeth by the number of pinion teeth. if your speedo slowed down you probably have 2.73 if it sped up maybe 3.42 or 3.73

 

[patracy]

Doing the math:

31" tall tire, stock 3.08 gear, 2K rpm = 60mph.
31" tall tire, 2.73 gear, 2K rpm = 68mph. 13% difference. 7.8mph difference of 60mph.
31" tall tire, 3.73 gear, 2K rpm = 49mph. 22% difference. 13.2mph difference of 60mph.

I'm going to guess 3.73's at that's closest to your noticed speed difference. While the engine might be turning a little more, it's not having to lug as much. 3.08's were too tall IMHO for these trucks.

 

[patracy]

Oh and I should have better explained myself.

2.73's would have made the speedo 7.8mph slower at 60mph.
3.73's would have made the speedo 13.2mph faster at 60mph.

 

[hogtruck]

If you want to go back to stock, send a PM to me. Have set taken off M1009. In 39051.

 

[tankie88]

I had the rear locking diff go on me a few years back(some teeth missing).I ripped it out and slung it on the junk pile.I now have a same ratio normal diff( 3.08 ) in.Don't ask what it came from as i don't know.I just phoned a junk yard and told him what ratio i wanted and for what vehicle.

 

[acesneights1]

The half ton diffs are junk. Take both those diffs and use them for boat anchors. Find a set of 8 lug 3/4 ton diffs out of any chevy square body and they will bolt right in. easiest swap you'll ever do and you will be much happier. I hate c clips diffs. No fun when the spiders come apart then the wheel and axle passes you...ask me how I know...This was my old Jimmy. Granted it's a civvy model but the principle is the same...And yes it was a 6.2

 

[BigDuck]

Sorry, first question faster or slower.... Answer faster, thank you patracy, 3.73's it is !! Second question how hard of a jos was it..... Not too bad at all ,4 ubolts , brake line , e- brake , and undo the u-joint and replace

 

[Matt1031]

What ratio do you want to use? For a 6.2 Blazer on stock size tires that doesn't need to haul anything heavy I would think around a 3.55 would be just about right.

First thing I do when I get a used rear is to pull the cover anyway -- to visually inspect gear condition, see if there's a locker or limited slip, check backlash etc. I usually double-check the ratio while cover's off.

While the diff cover is off, look for the P/N stamp the outer edge of the ring gear while rotating it- you'll see some small numbers stamped on it that will stand out if you know to look for them (you may have to wipe away black gear oil sludge if it's an old rear). If you see "41-11" or 41/11" or "xxxx[space]41[space]11[space]"that a 3.73 set. "41-10" would be a 4.10 ratio, "41-9" would be 4.56, you get the picture.

With the common light truck rear end ratios, the ring gear number is going to fall between 37 and 44. As you may have noted by above example, 41 is a relatively common number to see. Take whatever ring gear number you find and divide it by the smaller number that follows, which is the pinion tooth count. That's what ratio you have. It's easy to figure out once you learn to recognize what/where to look for that stamp.

Now that you know your rear end ratio, what are you going to do with the front axle? What year truck did it come out of? Dana44 or 10 bolt? Do you plan to run it off road and abuse the heck out of it? haul anything heavy? Or do you just want it to match the rear's ratio and get decent highway speed/mpg?

Your gas mileage probably improved because your engine doesn't have to work as hard now with the improved gearing. Even though rpm's are higher, your 6.2 is under a lighter load, so mpg goes up. It seems like you have accidentally arrived on a pretty optimal ratio for your driving needs. Slight reduction in top speed, but improved performance everywhere else. That's a "happy accident" if you ask me.

If you want the speedo to read your new corrected speed, it's pretty simple fix. Just swap out the plastic speedo drive gears at the T-case with the correct ratio. Much cheaper than moving up to oversize tires in order to make the speedo "think" you still have 3.07 gears.

 

[patracy]

3.55 or 3.54 doesn't exist for GM (those are Ford and Chrysler ratios). Only a 3.42.

 

[acesneights1]

I would Get 8 lugs. You can buy a whole rotted chevy truck for what that rear alone will cost you and the factory 10 bolt gov lok diffs are garbage. They always blow apart.

 

[ODdave]

I would Get 8 lugs. You can buy a whole rotted chevy truck for what that rear alone will cost you and the factory 10 bolt gov lok diffs are garbage. They always blow apart.

What are you planing on getting in that rust chevy truck thats better than a gov-loc?

Eight lugs are good if you need added weight. Use a 6 lug 9.5 w/ lock rite. Lighter than a FF14bolt and very strong.

 

[Matt1031]

3.55 or 3.54 doesn't exist for GM (those are Ford and Chrysler ratios). Only a 3.42.

Good call, my bad.

 

[jimmy-90]

I'd have to agree with MATT1031 about 3.73 gears being as good as it gets. Any lower and you would be wanting an overdrive trans. And about a 14 bolt ff and dana 60 to replace the 10 bolts all I can really say about that is that you can't find a set of axles like that for $250.00 lol. Im in the process of doing that to my 1990 GMC jimmy. I got a 14 bolt ff with a detroit locker out of another cucv for a fair price but the king pin dana 60 is gonna cost me an arm and a leg. I havn't been able to find one in decent shape for less than $800.00 Also my jimmy is not a street vehicle any more. I'd say yeah go ahead and just put the10 bolt in the front with matching 3.73 gears and enjoy being able to pull out in traffic quicker. Should run great that way.

 

[IdahoPlowboy]

The heavy duty stuff around me has been sold as scrap bought by the rock crawler folks or they want a high price for them. I see 10 bolt stuff all the time and for cheap, If it was me I would stay 10 bolt unless I was running big tires and digging hard. But after having said that I would really like to have a dana 60 and 14 bolt in my 09.

 

[acesneights1]

What are you planing on getting in that rust chevy truck thats better than a gov-loc?

Eight lugs are good if you need added weight. Use a 6 lug 9.5 w/ lock rite. Lighter than a FF14bolt and very strong.

I had two blow apart and I lost the wheel and axles. No thanks. 8 lugs for me from now on not to mention way better brakes with the 13 inch shoes. You put a GM 14 bolt with a Detroit Locker and now you got something....
I'd never own a c clip diff ever again. The gov lock used in the 10 bolts are not strong at all. In fact the open carriers are stronger. Now move up to a 14 bolt....You ain't bustin that for sure.

 

[patracy]

A gov-lock anything sucks. Even the gov-lock 14B FF rear. Now a detroit locker that is found in the CUCV 1008's and other truck chassis, that's the ones you want.

I've got a pickle jar filled with kibbles and bits from a gov-lock 14B FF diff that I blew apart.