Unfortunate News

[81GMCSierra1500]

The hip surgery on my 1981 GMC Sierra Classic is complete... Was assured by the shop that the gear ratios were identical, the diff is basically mint, whih is good, but I now cannot drive about 90ish km/h (55mp/h) without running over 3500rpm. I used to be able to cruise the highways easily at 110km/h (70mp/h)... Just dropped $800 into a truck that I cannot use... My only option as of now is to try to find an O/D trans (700R4), or sell it, as I will not be able to practically drive the vehicle long distances as it used to... Anybody have any ideas? I'm somewhat desperate, and very disappointed... By the way, the gears in the new differential are 3.08, so I either had 2.56/2.73 in the old 12 bolt that came out. (I still don't know what happened inside that diff)... Let me know your thoughts! I really would rather not sell it, but if I have to, I have to... :'(

[enaberif]

What tire size are you running to push 3500rpm?!

[81GMCSierra1500]

That is the strangest thing, I'm running 31" tires on the rear (very big tires for a 2WD), had I been running factory, I'd be pushing 4500rpm...

[hatzie]

If you didn't change the speedometer gears when you made the ratio change in the diff you are not travelling 80 km/h at 3500rpm.

What size are your tires? 24.5cm smaller or larger tires will make more of a difference than you might think in the road speed shown on the speedo vs actual road speed.

I have a spreadsheet near the bottom of the 73-91 manuals link that calculates road speed at specific RPMs when you enter the transmission gear ratios & rear gears along with tire size.

It'll also calculate ideal speedometer gears and ratio box percentages if needed.

[tucsonjwt]

I would verify the rear axle ratio first.

http://www.how-to-build-hotrods.com/gear-ratio.html

[rs74]

Originally Posted by tucsonjwt

I would verify the rear axle ratio first.

http://www.how-to-build-hotrods.com/gear-ratio.html

^^^ Yep I agree.

[beamn7]

I run 31" tires on both of my 2WD's. When I had the 3:08's and a THM400 I used to average around 2300-2400 at 65mph. When I switched to 3:73's I was at approx 2800-3000 (running off memory here..) I then swapped back in a 700r4 and now I average 1900 at 65mph.
I'd be willing to be you do not have 3:08's in that replacement diff....

[Dead Parrot]

Use a hand held gizmo with GPS to find out your actual speed. Note how off your speedo is from the GPS speed. Then go back to the shop and ask why. Could be something as simple as they put the wrong speedometer gear in or forgot they needed to change it. Or they changed it and didn't need to. They wouldn't need to if the gear ratios and tire sizes stayed the same.

Could be something as irritating as the new rear end being mis-identified. Maybe either the PO of your truck swapped gears or the PO of the rear end swapped gears and no one caught the switch.

[81GMCSierra1500]

And I did a ring-to-pinion gear rotation check (mark with sharpie on the yoke and the ring, and spin it. I most definitely have a 3.08 in the 10-bolt I put in, I'm thinking that the 12 bolt I had in it had 2.56 gears, judging by the absurd RPM difference at the same speed... I'll tell you something though, she's quick off the line, it just needs the extra gear... Anybody have a 700R4 they're not using I could trade my TH350 for? The transmission has 2000km's on it since a complete rebuild, with a shift kit

[tucsonjwt]

Something seems odd to me. In a stock set up, I would not describe a 3.08 as quick off the line. Maybe if you did have a 2.56 originally, then 3.08 would seem quick in comparison?

[81GMCSierra1500]

As I said initially, the shop was requested to verify both rear axle ratios before the swap was even started. I had already done a verification of the new differential at 3.08 (verification was done with all spider gears removed). The shop never contacted me about the ratio of the old 12 bolt, so I assumed they were the same. I'm quite irritated at this shop, for not doing what I requested before the work was started, they basically conned me out of $650... But I must have had a 2.56 in the 12, because, I'll say this, the truck before the swap, ran at 2300rpm at 110km/h (GPS Speed) with the original 12 bolt differential. At 110km/h with the new differential, it runs at about 3500-3750rpm. The speed was already inaccurate before the swap, because it didn't have the original transmission (the new one was calibrated for the standard 15's with 235's). I'm just angry because of the fact that I need this truck to be able to travel 110km/h and not be at the peak of it's power range, like it was before, the 3.08 ratio is just not gonna cut it... There's virtually no difference it I am travelling under 50km/h, anywhere above that an it is just revving too high... I'm just looking to see if anybody would be willing to do me a huge favor if they had a 700R4 in working order laying around...

[UKNOWME]

I have a speedometer app on my phone.

[81GMCSierra1500]

I already have done that... the gauge is maxed out (140km/h) when doing 110... I'm pretty sure I'm smart enough to do that :P My local GM dealer is useless. I got them to photocopy a catalog with color codes + part numbers, so when I pull the gear out, I'll no how far up in teeth it has to go. It already has a reduction box on the trans, so I'll remove that first and see what happens, then look at the gear. But my biggest concern is obviously, will running my engine at 3500rpm for 1-2 hours on end cause damage?

[SkinnyG]

That is so weird. I have 3.07 gears in my truck, 28" tall tires, and a TH350, and I'm doing about 2300 rpm at 100km/h.

[81GMCSierra1500]

Originally Posted by SkinnyG

That is so weird. I have 3.07 gears in my truck, 28" tall tires, and a TH350, and I'm doing about 2300 rpm at 100km/h.

Those are 28's though, 3" makes a fair difference... Another lame question, does a TH350 have overdrive? Maybe I thought it isn't kicking into overdrive?

[Liberty]

Originally Posted by 81GMCSierra1500

Those are 28's though, 3" makes a fair difference... Another lame question, does a TH350 have overdrive? Maybe I thought it isn't kicking into overdrive?

The TH350 does not have overdrive.
Yours being a 1981 WILL have a locking torque converter though.
As a TH350C.

[csdineley]

Something isn't right. My truck has a sm465 (1:1 final gear same as a th350), 31" tires and 4.10 gears and at 55mph it runs 2400 rpm. To hit 3500-3700 I'd be doing 80-85 mph.

You sure your transmission is not stuck in 2nd gear.

[81GMCSierra1500]

Indeed something isn't right... Any ideas? Bad torque converter? (I can get another for nothing), Trans issue? I never expected such a drastic change in RPM... One notable change, the diff-to-shaft U joint had to be changed to a "combination" U joint... The yoke on the diff is a bit wider than the yoke on the driveline, and the diff did come out of an 87 4.3 V6 Standard (maybe 4-speed?)... Does this change the situation?

[81GMCSierra1500]

And yes, It is shifting from 1st through to 3rd...

[SkinnyG]

3.08 gears, multiplied by 1.62 second gear ratio, running a 31" tire, would result in 3515 rpm at 62.5mph (100km/h).

I'm betting you're in 2nd.

On your shifter, is reverse where reverse should be, neutral where neutral should be, and Drive where drive should be?

[81GMCSierra1500]

Good thought, I might just have to go out and check, but all the TH350 transmissions run a PRND21 shifter, so 1-2-3 clicks to Drive. One thing to note, it has always shifted rather hard into drive, and very lightly in to reverse. Is the linkage out of adjustment? What are your thoughts?

[SkinnyG]

4.88's in a real operational 3rd gear, with 31" tires, would give some VERY snappy acceleration, and 3438rpm and 100km/h.

[franken]

The number of teeth is stamped on the ring gear. 41-10 means 41 ring and 10 pinion. Pull the cover off both and get the numbers. This should have been done before anything.
Shifting from 1st to 3rd? My automatics shift from 1st to 2nd. Typo, or?
I'd guess the modulator hose isn't connected and its in 2nd.

[81GMCSierra1500]

Originally Posted by franken

The number of teeth is stamped on the ring gear. 41-10 means 41 ring and 10 pinion. Pull the cover off both and get the numbers. This should have been done before anything.
Shifting from 1st to 3rd? My automatics shift from 1st to 2nd. Typo, or?
I'd guess the modulator hose isn't connected and its in 2nd.

We did that, and the numbers corresponded with 3.08 gears, we double checked using the spin the yoke method and they were indeed.

[csdineley]

Either the rear end gears are not actually 3.08's or your trans isn't not shifting into 3rd. That's the only two things that make the most amount of sense. Why it wouldn't shift into 3rd is another whole story. Sorry Auto's are not my thing.