I've been attempting to fix this car up. I bought it for 700 through an ad off of craigslist.org and this is what I have found out so far. It has a 305 with 60000 original miles, and a turbo 350. The rear I don't believe is posi because when I jack up the rear and turn one tire the other turns the opposite direction. I also don't know what Gears I have in the 8.5 10 bolt differential. the transmission shop said that I may have 4.10's. My speedo is off but I followed my brother in the car in front of me and found that I was pulling 3200 rpms at 40 mph and can, at redline, hit 55. What is wrong with it? I did the calculations and found that my gear ratio, including transmission, was 5.9. so in third that would mean that I have a 5.9 rear end correct? Are there gears that low?

Any help would be GREATLY appreciated.
Tim C.

 

That doesnt seem right.. are you sure thats in third gear? 1:1 ratio third? assuming a standard tire around 25.5", you would need a gear ratio around 5.2 in that rear end. Just wouldn't make sense.

 

I am sure that It is in third, but I don't know what the ratio is, I assumed that it is 1:1. I have a 12" tire radius 24" diameter. Also, I attempted to find the rear end gear ratio bu jacking it up and turning the driveshaft and came up with 6.5 and didn't think that was correct. The other thing is that my kick down cable is not connected yet. would that cause this?

 

We need to know what we are dealing with first.

-What year is the car?

-No third gen ever came with a Turbo 350. At 60,000 miles it's doubtful that it's been replaced. A TH200 3 speed was standard in 82/83 and the TH700R4 4 speed automatic was standard after that. How many speeds do you have? How do you know that you have a turbo 350? If your transmission pan says METRIC on it, you have a 200. If your car was swapped to a TH350, that certainly explains why your speedometer is off. Your tach may be off as well, they are not the most accurate gauges available.

-No third gen ever came with an 8.5 ten bolt. If you have an original looking rear end (meaning that there are no brackets welded to it to make it fit where it doesn't belong) it will most likely be a 7.5 ten bolt. Believe me when I tell you that there is no 5.anything in any 7.5 ten bolt around and certainly not in your car. Forget your calculations and do this to get close. Jack up the back of the car, with your rear axle on jack stands, and put the car in neutral. Put a chalk mark on the driveshaft at the bottom. Put a chalk mark on both tires at the bottom. Now, with a friend, rotate both tires in the same direction once until the chalk mark goes back to the bottom. How many times did the driveshaft turn around? It will either be less than three times or more than three times. You'll get close. Was it 2 and 3/4 times? Probably 2.73. Was it 3 and half times? Probably 3.54. You get the idea.

Start there and report back!

 

It is an 83 Z28. A transmission shop looked at the pan and told me it was a 350, and gave me the gasket and it fit so I assumed he was correct. 3 speeds, unless it's not shifting into the last one, which the transmission shop told me that it was shifting strong and in all gears. i will check the rotations and the rear end to see if it is stock and let you know what I find. Thanks for the help.

 

Everything looks stock and based on your test it looks as though I have a 3.23 based on 3 and a quarter turns.

 

Ok, so after driving it today, It seems as though it doesn't shift into third. Starting off in first It will sometimes shift twice but on the second shift the rpm's don't drop but maybe a 200 rpms so would the kickdown cable be affecting the shift into third?

 

The reason I was asking about it being a "real" TH350 is because of the shifting problem. If you had the original 200, the tv cable actually sets the shifts. In a TH350 you have a vacuum modulator. If yours is old or screwed up, it will do what you describe. Also be sure to check your vacuum line going to it and where it hooks to your intake. A vacuum leak can add to the problem.

Your tv cable for a TH350 is really just a kickdown cable. It should shift fine without it.

So, either your vacuum modulator is not working right, or you have serious internal problems - better check that modulator.

 

All that seems to be fine and plus, when I put it in low gear it shifts from first to second, then I can put in drive and I can tell that it shifts into third. How would I check the tach gauge to see if it is reading accurately?

 

You'll have to hook up another aftermarket tach and watch both needles to see where you are with your tach.

In my old cars with TH400/TH375/TH350 transmissions, when it wouldn't shift sometimes, but then it would if you popped it down to second and then put it back up in third, meant the modulator was bad or the line to it was leaking.

You also need to take the line off of the modulator and look inside the hole. Sometimes there is an adjustment screw that you can monkey with.

 

Thanks for your help. It seems that it was just the Tach running at almost double the actual rpm's
. Now all that's left is to change the transmission mounts and I've found the tech article on that.