69 Rear end Ratio ??

[a69vette]

Can anybody help me with this one? A '69 w350/350 4 spd. When running about 70 mph, the tach is @3500. Anyway to tell what ratio that maybe? Second, is it possible to install a lower gear (lower RPM @ higher MPH) that would still be numbers correct???
Thanks, Rick

 

[Rowdy1]

Depends

If both your tach & speedo are correct with stock tire size, I'd guess you have nothing less than a 3.90 maybe a 4.11. I used to jack them up and count tire revolutions to the driveshaft to get an approximate idea of what ratio it was.

 

[a69vette]

Tach and speedo are correct, tire size is very close to orignal. A friend had suggested he thought it might be 4.11 also. Now I like to know if, and how I can make it a little more road friendly. I don't want to destroy any originality.
Rick

 

[ChuckG]

As a "rule of thumb" only, with stock diameter tires, if you "double your tach" in 4th. gear, you have 4.11 gears. 3500RPM = 70 MPH, 4000RPM = 80 MPH, and so on. Jack it up and count the driveshaft revolutions per one rear tire revolution. If the driveshaft turns 4 times per 1 tire rotation, you definitely have 4.11's. You could also check the assembly stamping on the rear end housing and see what the original ratio was. The ratio could have been changed, or you might have original 4.11 gears. Hope this helps. Have 4.11's in my 63. MISERABLE ratio for highway driving, great around town. Chuck

 

[JohnZ]

Chuck's method will nail it down for you. My '69 Z/28 had the original (optional) 4.10 gears, and turned up 3800 at 70. I swapped out the 4.10's for 3.55's, and now it turns 3200 at 70 (235/60-15 tires, 26.1" tall). Based on this, you may have 3.70's, which were originally available only with the close-ratio 4-speed.

 

[Eric]

Here's another thread - same question, similar answers. Good luck!
http://www.corvetteactioncenter.com/forums/showthread.php?s=&threadid=3950

 

[rpounds]

The JohnZ that just posted is apparently a new member to this board. However, I am familiar with him from another board and he is well worth listening to.

John H . . . welcome . . . I am personally tickled to see you here.

Ron

 

[garin1]

have been gone from the forum for a while.i have a 68 and found that if you have 4:11 gears and a bad clutchpack in the rear end you will not get an accurate reading.look for the stamping on the bottum of the diff housing.also,you can not go smaller gears without changing the cltchpack(very pricy).

 

[tls630]

69 rear

I have a 69 350-350 and close ratio m-21 4 speed. I run about 3550 at 70 mph. Mine is a 4: 11 RATIO. Iwant to go to a 3:36 or 3:55 that would make it a lot more road friendly, it is just to low to enjoy now. I will save the 4: for the next owner.
Hope this helps.
You did not say what tranny you have?
Tom

 

[Red69]

A guy who owns a Corvette shop near me said, if your tach and speedometer are working properly and you have a 4:11 rear gear; your mph will be double your rpm's. {(CORRECTION) Sorry, that is suppose to be MPH will be double the first two digits of your RPM's.} My 69 has this gear and I figuer the best of both worlds is had with a 5 speed trans. You get good accleration with a .83 overdrive gear for highway cruising. I expect to be doing the swap before the year is out. If you don't mind losing the low end performance a 3:08 rear gear is a viable alternative IMO.

 

[wallyknoch]

70 MPH at 3500 RPM`s indicate it`s a 4.10 gear. A 4.56 gear will read 4000 at 70. I use them both in my Vettes and Camaros. The 62 below is equiped with the 4.56.

 

[Dave41052]

For originality's sake, you only have one choice - stay with what your tank sticker says. Factory Standard for 350hp, with M21 4 speed was 3.70, performance option is 4.11

 

[Artsvette73]

ratio

I stand to be corrected,but if you have 411 ratio.and wanted to drop down say

355 you have to change the carrier to a 3 series I forget the break off point
For the 4 series carrier

 

[tls630]

Dave
I planned on keeping the 4:11. Just put in the 3:55 or 3:336 for my own use. Then if I sell it both will go with the car.I would think that would make the car worth more. I do not plan on having it judged, it is not of ncrs quality.Its A nice car but I am not going to go to that extreme, I would be afraid to drive it if I got it to that point.Its # matching except for the alternator and I like it. Thanks for you input.
Tom