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I think that may be the underlying issue. (3rd gear) Has anyone seen third gear? who remembers that commercial (circa 1978) from Aamco?

 

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Radovette,

Keep in mind that the tag on the differential represents the ratio that WAS there from the factory.
It does not guarantee, that is the current ratio. It's been 40 yrs. A lot can happen.
You can read and trust that axle tag with the same faith as a chrome air cleaner decal that says: 375 Horse.

You suspect a 3.08 ratio. I suspect a 3.31 or whatever is next in line.
Trying to narrow down ratios with such a small difference in how far the pinion yoke rotates is impossible. Just a fraction of an inch circular movement of the yoke,
represents a lot of difference in a true ratio calculation.

 

To get 55 mph with a tire that is 26.6" in diameter at an rpm of 2,750 would calculate to 3.90 rear. One of your measurements is wrong.

 

Here is the math, assuming 26.6 inch dia rear tires:

26.6 in X pi = 83.5 in = 6.96 ft tire circumference

55 mph = 5,280 X 55 = 290,400 ft/hr = 4,840 ft/minute

4,840 ft/min / 6.96 ft / revolution = 695.4 rev / minute Tire revolutions

695.4 tire rev / min X 3.08 engine rev / tire rev = 2,141 engine rpm

Adding a little converter slip, maybe revving 2,300 at 55 mph with 3.08 rear gears

 

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Why you have to be such a jerk to everyone on here?
Your attitude & behavior is not acceptable.
Move on to another forum that enjoys your rudeness. We don't.

 

. From reading replies here,,, seems the experts don't think it is extreme torque converter slippage......
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I've been driving these cars my whole life.
Yes, since they were new.
Yes, these RPM'S are PERFECTLY NORMAL! It's your F, in on line calculators that are garbage!
ENOUGH of this rubbish! You want a vintage car to turn RPM's like a modern car and go mental when it doesn't!
ENOUGH!

 

Excellent description of the procedure I've used hundreds of times at salvage yards laying on the dirt. I never missed one.
Although I never thought to use chalk. The inside of each tire will have some distinctive marking for a handy index. I would rotate the pinion until my chosen tire index aligns with anything else; use rock to scratch bottom of drive shaft. Then, check the other tire for a similar opportunity. I would rotate the pinion 1 full turn, then check that both tires remained aligned. Then rotate 2 more full turns, to note positions of each. The final twist then finished my measurement to verify the tag.

This particular expert is not so convinced the TC and TH350 have not conspired against you here. I'm just guessing, but I have to doubt those are both original. These must match or the symptom you described is the exact result expected. In late 79 the TH350C was born which hydraulically actuated a lockup TC. Later, later, LUTCs went electric. If these components are mismatched, it'll not fail but keep your tranny in sport mode forever. If it's been driven like this a few weeks, then I would doubt tranny failure, because it would already have fully failed. Whomever suggested sniff the stick was running down that path. That's surely quick and easy to check.

I never have much luck with diagnosis from afar so I'm not saying this is it. But the evidence you presented, eliminates rearend ratio.

The conventional TH350 had part codes of M33, M38 & M39. The TH350C had codes of MV4, MX2, MX3 & MX5.

If your tranny and TC part numbers are correct, there are several other issues which plague GM cars. For example, If the tranny quit 20 years ago, the owner or a shop may have sourced any ole TH350 maybe with a C suffix, maybe not. The TH350 was extremely popular. Buick even co-developed. While the casings are nearly identical, the internals vary according to vehicle weight and engine power. So, if your car has a tranny from a V6 engine, it may never measure conditions to
lockup. If you now have a TH350C tranny which is not connected to your brake and thermostat, it'll never lockup. The TH350Cs typically came with a higher stall TC, because it could lockup instead of compromising with increased coupling.

Now that I chattered in circles, I'm sure it makes less sense to everyone.

 

So reading your thread got me curious about "Kandi's" rpms so I had to check it. At 45mph she is at 1700, 55mph 2000, 65mph 2400, and 75mph is 2750

 

This thread has ran it's course and should be closed. All the possibilities have been mentioned along with some very obnoxious posts.

 

To all who responded to Radovette—thanks for your input. I now have some more homework to do to narrow this down. I’ll report back on a new thread with my discovery if any. As many have said in the past, the collective wisdom here is a wonderful thing. Radovette