The guy I bought the car from said he had put 3.90s in the rear but "got in trouble and wanted better gas milage" so put some 3.50s in. Basically, I'm not sure If I believe him.... I have a 305 TBI with a 700r4. At 70mph in OD, its at around 2000rpm(stock converter). It seems to friendly for 3.50s (which are extremely hard to find for our rears). Any ways, I'm thinkin I have the stock pegleg 2.73s. I'm actually hoping I do so I know I can gain by going bigger to say4.10s with my T56.

I know there's a calculator on this sight for it, but there seems to always be two or more pieces of info I'm missing.

 

I've got 3.08s in my Berlinetta. I turn about 2500 rpm at 70 mph. So 2.73s sound about right. Hey, better gas mileage, right?

 

I have 3.27 and I drive on 60 mph at 2000rpm's

 

Sweet! that's what I was hoping for!!!! It most likely means I do have the pegleg back there, and will gain that much more with 4.10s when I finally get that 6 speed installed. Thanks for the inputs!!!

 

I tach 2500 rpms @70 with 3:73s. An easy way to check the gears do 70 mph in D, if your tach reads 3500 rpms, then you have 3:50 gears and so on.
Oh, and 4.10s will most likely make your car slower, if the motor is stock.
And pegleg refers to a non-posi whatever that is referred as. Not really pertaining to gears, as far as I know.

 

Are you sure? sounds like you have 3:73s, I turn 2500 rpms at 70 in OD, with a 25.5" tall tire.

 

Well, I have to have 3.08s. That's the only option that came with the 84 Berlinetta w/305 LG4 engine. And trust me, that rear end looks stock, like the cover has NEVER been taken off. It's on my To-Do list.

 

Get dirty and find out for sure. Here's how: With the car safely up on stands, mark both tires at the inner bottom and mark the driveshaft pointing down. Now count the number of turns of the driveshaft as you turn BOTH tires TOGETHER, until they have completed one full revolution. Break it down into quarters. 3.08 will be just slightly over three turns where 3.23 will look like three and a quarter turns. 3.73's will look like three and three quarters turns and so on. Now you know beyond a shadow of a doubt.

 

I just checked and it's 2700 rpm exactly at 69 mph. Hope this clears things up.

 

Unless you have really short tires or a tach that is way off, I'd say you don't have 3:08s.

 

I appreciate everybodies responses! I tried to third gear test at 70 mph and it was about 2900 rpm give or take. I could be off a little, I just replaced my gauge cluster with one from a '92.....the speedos a little different (65mph WAS staight up before and now it is a little to the right of that....about where 67mph would be on my old one), so I could be going a few mph faster than it reads, which would be why it was at 2730rpm at 70 mph.....
Regardless, my rear is stock differential with a sad rear gear.
This is actually good news.....If there were 3.50s in there, then it would have meant my engine really sucked. I a happy man.....that is until I have to pay for the new rear and labor.....

 

Well, the Berlie is all digital. I believe digital stuff over analog any day. Any way I can tell the ratio for sure by looking at cast numbers on the differential?

I highly doubt the car was modified at all. The engine is all stock right down to the air pump and tubes, whitewall tires on 14" rims, and original rusted out exhaust.

 

If Rpm's at 70mph in 3rd gear are supposed to represend rear end gear, then why does my tach only read 2350 rpm's at 70 mph in 3rd?

 

factory tachs are usually off, or go bad.

 

FYI, I've got a bone stock '91 RS with the 305 and 700R4 and I barely turn over 2k at 70-75 in OD and I have 2.73s with 245/50-16s.

I'd bet you still have the 2.73s too.

Also, 3.90 and 3.50 gear ratios for a 10 bolt?? Come on...
Correct me if I'm wrong.

 

The rpm's at 70mph will only work if you have a certain size tire, and the speedo and tach are really accurate. If you post some tire sizes, the comparison will be much more accurate. If you want to be sure, pop the cover and count the teeth or look for the numbers on the ring gear itself. Or spin the tires one turn and count the driveshaft turns.

 

Thanks for all the responses everybody! I just ordered the SLP take off Posi, along with a new differential cover and 3.73s for the three series carier to work w/ the posi. Still need to get an instalation kit. Any ways, I can't wait!!!!
For the longest time, I never even believed the guy when he told me it had 3.50s, because I had never seen them for a 10 bolt let alone a third gen. One day when I was looking into differentials and ring and pinions, I came across a web sight that actually had them. Kinda worried me 'cause it would've meant my engine was really shot if it had 3.50s and launched the way it does. But they are out there eb, believe it or not.

 

Why? Digital is only as accurate as the sampling frequency and hence is discrete in time whereas analog is continuous. Just curious.

Miles

 

Miles,

I believe digital over analog because over the span of 18 years (for me, anyway) the sampling frequency doesn't change but the analog mechanical stuff wears out and the magnet (if they still make them that way) can go weak.