Something ain't right - new gears in rear end
 

I've got a 78 K15 with (as far as I know) the original SM465, NP205 and 3.08 gears. I went up to 33" tires during my build, so gears weren't quite right to handle various situations - right turns, etc.

About 7 years ago, I had some axles rebuilt professionally with 4.10 gears and limited slip differentials for another project that got put off. I also have an NV4500 and my plan is to swap these axles and NV4500 into my '78. This past weekend I started by swapping out the rear axle.

Pulled the 3.08 axle and transferred my fully rebuilt rear brakes intact - still mounted on backing plate - to the 4.10 axles. Installed new axle shafts into the 4.10 axle. Filled with limited slip additive and gear oil. Cover on and installed the axle onto the truck.

Now here's the interesting part/problem. I knew that I'd be running at higher RPMs because I haven't swapped out the sm465 yet, but I'm hitting the higher RPMs at lower speeds than the gear calculators suggest. I'm running 2500 RPM at 48 MPH in 4th gear. The gear calculators say that I should be doing 60 MPH at 2500 RPM. I tested my tach against the tach on my timing light and they matched. I'm using a GPS for a speedo so speed should be accurate. According to the calculator, my gears would be 5.12 in order to be doing 48 at 2500.

I suppose the drivetrain shop could have installed the wrong gears but before I jump to that conclusion I'm asking for suggestions on other possibilities I should check. Thanks --Rick

Re: Something ain't right - new gears in rear end
 

Pull the diff cover and look.

Re: Something ain't right - new gears in rear end
 

Your speedo is most likely reading under your actual speed and you need to swap the speedo gear in the t case due to the gear change
Posted via Mobile Device

Re: Something ain't right - new gears in rear end
 

Well, that's ultimately what I planned to do if nobody had any other suggestions to check first. Just didn't want to deal with having to drain the oil and additive until it was inevitable.

Re: Something ain't right - new gears in rear end
 

I was using a GPS for the speedo. I know that the speedo gear is wrong.

Re: Something ain't right - new gears in rear end
 

Never mind then. I'm bad at reading comprehension. I'd start counting teeth as was already sugggested. What kind of axles are these?
Posted via Mobile Device

Re: Something ain't right - new gears in rear end
 

If you have a posi, then jack up both back tires and spin one tire one revolution and count driveshaft turns. A little over 4 turns means 4:10 ratio. This confirms your gear ratio.
Now if you have a gov loc the procedure will be a little different.

Re: Something ain't right - new gears in rear end
 

with 4.10s...33s...and a sm465 I'm getting 2004rpm at 48 mph....
http://www.csgnetwork.com/multirpmcalc.html
sm is 1:1ratio in 4th
nv4500 is .73 in 5th

5.13 would give you that 2500rpm at 48mph....I'd pull cover and count teeth....
once you go to the nv4500 your 48mph rpm will be 1830 and 65 or so will be back at 2500rpm
I'm betting you've got 5.13s

Re: Something ain't right - new gears in rear end
 

I'm sure hoping you are wrong mongo. Don't want 5.13's - not even sure about 4.10's. Even if I paid for 4.10s and got 5.13s instead, the axles have been sitting in my shop for almost 7 years. Doubt the drivetrain shop would be excited about making it right at this point.

I'll at least give the tire spin, count driveshaft revolutions thing a try tonight when I get home. Thanks for that suggestion.

The axles I've got are Corporate 12-bolt rear and Dana-44 front. Will have to dig out paperwork to remind myself what limited slip diff I got - its been several years and quite a few more than several beers since I got them :lol:

Re: Something ain't right - new gears in rear end
 

Richmond gear apparently makes 5.13s for the 12 Bolt. Tooth counts are 41-8. I think 4.10s with the NV4500 would be perfect. I have an '83 K-10 with 3.73s and an SM465 on 33s and it drives great at 70 mph. Doesn't like going much faster though.

Good luck!
Posted via Mobile Device

Re: Something ain't right - new gears in rear end
 

Update with new info. Got home from work and jacked up the rear of the truck. I count 4.5 rotations of the driveshaft for 1 rotation of the tire. Based on 75c10's method, it could be a 4.10. FWIW, both tires spun in the same direction.

I also realized that I could count gear teeth on the front axle which hasn't been installed yet. There are 45 teeth on the ring gear and 11 on the pinion. Plus, stamped on the edge of the ring gear is "D44-409 11x45 1 11". As I understand it, I divide 45 / 11 = 4.09 and its pretty much confirmation that the front axle at least is what I paid for.

Found my receipt for the work done on the axles and basically all it said was "duragrip" for the differentials. I think that means Yukon Gear and Axle Dura Grip.

So, all indications seem to say I've got the 4.10's I thought I had. I could still pull the cover off the rear and confirm. But, I think I'm back to square one.

Re: Something ain't right - new gears in rear end
 

Or 4.56 gears...

Did they match the gears in the front and rear axles? I'd check before you engage the 4wd.

I don't bother with the calculators if I don't have to.
The GM parts books have many pages of tables that'll tell you what speedometer drive and driven gears GM used with your tire-size, axle-ratio, and transmission/transfer-case. The tables also note what ratio adapter box was used... if they used one with that combo.

All seven volumes covering the 73-91 CK, D, & RV squarebodies are in the manuals thread linked in my signature.

Re: Something ain't right - new gears in rear end
 

I haven't installed the front axle yet. Didn't plan to engage 4wd until I have. I know that the spedo gears are off and I plan to deal with that after I finish the rest of the drivetrain swap (front axle and the NV4500). As far as I know, the gears for front and rear are supposed to be matched. I'll verify by counting teeth on the rear gears when I get a chance.

Re: Something ain't right - new gears in rear end
 

[ I'm running 2500 RPM at 48 MPH in 4th gear]
for some reason the math doesn't work out to this being 4.10s


ive got 4.10s behind my nv with a 30" tire.....I love this combo behind my LS....
my current build is getting the LS/nv combo as well.....going with 33" tires and thinking 4.88 or 5.13 for it...the axles currently have 3.73s...I really like the way my truck responds to that combo and am gonna duplicate it....I hope

Re: Something ain't right - new gears in rear end
 

Right, that's whats got me scratching my head. Could too much or too little oil in the housing cause this?

Re: Something ain't right - new gears in rear end
 

GPS method of speed measurement is off. Find a stretch of road where you can drive 1 mile at 60 mph and see if your over or under 1 minute; most highways have mile-markers. Once you've eliminated that variable, just pull the cover and be done with it. An oil change never hurts.

Re: Something ain't right - new gears in rear end
 

Thanks, but I doubt that the GPS is off by 12MPH, and knowing my speedo is off I have no way other than GPS to determine that I'm driving 60 mph. The cover will be coming back off this weekend, but ouch the oil and additive are new as of last weekend!

Re: Something ain't right - new gears in rear end
 

I agree with Hatzie. If you did this several times and you observed a driveshaft rotation of 4 1/2 times for one rev of the wheel you have 4.56 gears. This method is never wrong as long as you are sure of the counts. The gears cant slip or jump and its a mechanical connection. Secondly if you only have low miles on the oil you can put it back in. Presuming this was cleaned before it was assembled there should be minimal debris in the oil. And if there is a bit of solids, just put back in most of the oil and leave the debris in the catch pan. Top it off with a bit of fresh fluid. If it looks ugly looking then replace it.

This gear ratio determination method also works when there is not a posi i.e an open rear end. Except you jack up just one rear wheel and spin it twice. You spin twice because of the spider gears. Place the valve stem at the bottom so you don't loose track of what you are doing.
Works every time. Careful with your drive shaft counts. This is a mechanical connection and will give you the same answer over and over.
There is no need to open the diff to determine the gear ratio.
Its a confidence thing. After you have done this a few times you will have faith in the test. Next time you open a rear end, check the ratio first then confirm by looking at the gears.
OBTW the ring gear and pinion are a matched set and the tooth numbers are usually stamped on the ring gear. It pretty rough to count the pinion gears when its assembled. Good luck

Re: Something ain't right - new gears in rear end
 

Well, on this rear axle I counted 41 teeth on the ring gear and 9 teeth on the pinion. My math says thats a 4.555... or basically not the 4.10 I paid for. I didn't see a stamp on the ring gear here, so I used a sharpie to mark a tooth on each and just counted.

So, I guess this mystery is solved. Now I will give the drivetrain shop a chance to make things right, but since its been 6-7 years, I'm guessing I'll be paying someone else to install 4.10s in this rear axle.

Thank you everyone for your suggestions and opinions!

Re: Something ain't right - new gears in rear end
 

I'd sure double check the ratio in the front axle after this find. ....

Re: Something ain't right - new gears in rear end
 

Ohhh Yeah... :dohh: You might've gotten what you paid for in the front axle or they might be 4.56 gears too. No telling without a looksee.

Re: Something ain't right - new gears in rear end
 

Thanks guys. I already did count teeth on the front axle (post #11) and that one has 4.10s.

Re: Something ain't right - new gears in rear end
 

Dang man that sucks.... I can't imagine having 4.56 with 33's. I have 4.10 and the 33's aren't cutting it, my next set of tires will for sure be 35's.