what rear end to I have

[justinj]

At 70mph I am turning 3300 rpm the previous owner told me it came with the high performance rear end is that a 3.55 or 3.70 or higher.

 

[81 Corvette]

At 70mph I am turning 3300 rpm the previous owner told me it came with the high performance rear end is that a 3.55 or 3.70 or higher.

3.7 or 3.73 or something in that area

3.55 would put you at about 3100 rpm

 

[mvftw]

I would say 3.55. I have 3.70's and at 70 i'm over 3500...There is a two letter code on the bottom, clean off the rust and check it...

 

[Fishman]

According to my calculations... all at 70 mph:

3:08 3000 rpm
3:55 3458 rpm
3:73 3633 rpm
4:11 4033 rpm

This is based upon a formula I was given a few months ago. It was accurate for my car.

So you most likely have 3:55's.

 

[81 Corvette]

one way to settle the debate....put the rear up on jack stands and put the car in neutral and then note the position of the drive shaft. Count the number of revolution the tires have to make to make the drive shaft turn once.

I'm still calling 3.7 cause i'm stubborn like that. lol

 

[Glensgages]

"How ya doin', Fishman???"

I agree: 3300 @ 70, with a 'common' 27" tall tire,would be an 'odd-ball' 3.43 ratio, which was not offered.
One source-book lists 3.55s with '79 automatics, and '.36s with 4-speeds.

 

[Fishman]

"How ya doin', Fishman???"

Hey Glensgages! Long time no speak. I hope all is well with you.

I am sooooo close to putting in the 3:73's. Shouldn't be more than a month or so. And thanks for all the calculations... you made me look (almost) smart. :L

 

[Glensgages]

And thanks for all the calculations... you made me look (almost) smart. :L

You stand close-enough to me very long..... and you WILL look smart!!!

 

[81 Corvette]

Here are my results

Engine RPM Rear Gear Tire RPM MPH
3,714 4.1 906 72.8
3,379 3.73 906 72.8
3,207 3.54 906 72.8
2,999 3.31 906 72.8
2,600 2.87 906 72.8


Assume 747 tire revolutions per mile based upon

tire circumfrance = 27 in tire times 3.14 = 84.78 inches divide by 12 = 7.065 feet

5280 ft per mile / 7.065 = 747 tire revolutions per mile

engine rpm divided by gear ratio = tire rpm

60/747*tire rpm = mph

where did i go wrong if i went wrong?

i've got it in excel if anyone wants to take a look at it.

i had 2.87 gears in mine rear and the engine rpm made sense.....

 

[photovette]

None of you asked if he has automatic transmission or manual--that'd make a big difference in rpm's due to slippage. Also, most rear-ends have been worked on or altered by this time; who knows what someone's put back in. The only way to really be sure is to, like was stated, jack the back end up and count the revolutions.

I'm having my car aligned as I'm writing this and I asked the mechanic to perform the revolution test for me. I have automatic and I pull 3000rpm's at 70mph (during the day) and 3300rpm's at 70mph at night. Don't ask me why there's a difference...

 

[Glensgages]

photovette:
you beat me to the punch regarding the automatic/stick.

When talking about a PG or THM350/400, you need to calculate approx 10% slippage due to the torque converter, and in most cases, a stick has no slippage.

I've used this for race-cars for years, and it is very accurate:
RPM x .006 (a factor) x .9 (10% slippage if auto) x 1/2 TH (tire height) div/by GR (gear ratio) = MPH

3300 (RPM) x .006 x .9 (slippage) x 13.5 (1/2 TH) div/by 70 (MPH) = 3.436 (GR)

Most people are smart-enough to figure this out, but fail to consider the slippage in the converter.

 

[81 Corvette]

yet anouther reason to hate automatics.

I didn't even think about slippage as mine has a torque converter that locks up, which eliminates teh slippage and makes my calculation make sense for my 81

soooo, does that mean we are all right?

 

[photovette]

Considering that I have a different rpm reading by day than by night, I don't trust my tach to be accurate enough. After all, a 400rpm tach error would make a difference of having a 3.08 to a 3.55 rear. I am looking forward to the mechanics reply and answer in the morning--I've been wondering for several years now.

I just discovered that one of my u-joints melted to the flange--I wondered what the squeaking from the back end was...

 

[81 Corvette]

Melted to the flange....well that can't be good

If it makes you feel any better, teh shop doing my rear end rebuild (see installing 3.54 gears :jester busted off one of the lower shock mount holes off the rear spindle support

 

[photovette]

I got lucky though; I had purchased a single 'vette 1/2 shaft on Ebay to mess with--it had the flange I needed still attached to it--only cost me $10.

 

[justinj]

Must be a 3.55 the owner has replaced the speedometer cable once before and the tires are 60 series not 70 series so I am probably off by a couple hundred rpms.

 

[photovette]

okay--60 series, automatic, 3000rpm at 70mph--the shop counted the revolutions--I have a 3.55 for certain.