400hp427vette

I haven't looked at the rear end of the car (1969) to pull the numbers off it. When I do where are they located? Also the gear in the M-21 transmission is a 22 tooth green speedo gear. What ratio is the rear end?

maddmickey

If the 22 tooth gear was used with a 8 tooth drive gear 3.55. If it was used with a 7 tooth drive gear 4.11. What is your cruising RPM at 60 or 70 MPH? If you know that I bet we can tell you.

400hp427vette

damn, I wont be able to tell for atleast a week. I will do the and reply once I know for sure

lakebumm

HI BRUCE

I FOUND THE REAREND CODE ON MY'66 ON THE BOTTOM OF THE GEARCASE (PUMPKIN)

THERE SHOULD BE A DATE CODE CAST INTO THE REAR HOUSING. FIGURE OUT THE DATE AND LOOK UP THE CODE TO FIND AT LEAST WHAT RATIO CAME IN THE REAR.



BILL SR

FKING1

Code # will be cast on the case.
1969 rear end ratios:
2.73 - AY
3.08 - AL,AR,AT,AU,AW
3.36 - AM, AU
3.55 - AN, AZ
3.7 - AO, AS,FA
4.11 - AP, FB
4.56 - FC

early shark

400hp427vette,

If you indeed have a factory installed M21 transmission, the std gear ratio is 3:36 ratio. 3:08 would be the optional economy ratio and 3:55 would be the optional performance ratio.

If your L-68 is equipped with an M20 transmission, 3:08 is the std ratio and 3:36 ratio would be the only performance ratio available.

400hp427vette

Thanks for all the help, Also I have read that late builds had 1970 coded rearends My car was built 09-22-69 could it have a 1970 coded rear?

Binnie77

Yes, 1969 models built after approximately August of 1969 use 1970 coded differentials so yours is no doubt a 1970 code. 1970 used 3 letter coding where as 1969 used only 2 letters.

wfo76

late 69 and 70 big block codes

2.73 - CAY
3.08 - CAT, CAV, CAW
3.36 - CAU, CAX, CLR
3.55 - CAZ
3.70 - CFA
4.11 - CFB
4.56 - CFC

OldCorvetteKid

This may seem to easy but...if you look what your RPM's are at 70 mph, its usually the gear ratio....at 1.1 (not overdrive).

400hp427vette

so if gear ratio is 3.55 tach at 70mph would read 3550?

RIJVETTES

Not necessarily true. Check your tire diameter and use the formula from Chevy Hi Performanc to calculate rear gear ratio= RPM X tire diameter/ MPH X 336.

OzzyTom

????
What you are suggesting is not correct for typical rear tyre sizes on Corvettes
It might hold true if you had 185/60series tyres.

wfo76

On the C3 with 27"-27.4" tires, it's more like 80 MPH, not 70.

Try it, it works. 4th gear at 80 mph will read the gear ratio. 3000 rpm at 80 is a 3.08, 3300 rpm at 80 is a 3.36, etc. The mid year cars with the smaller tires work out best at 75 MPH, but you would need donut wheels for this to work at 70.



DISCLAIMER: This assumes a manual transmission. With an automatic with non-lockup converter, add some rpm for converter slip.

400hp427vette

taking the car for a ride saturday will let everyone know what i come up with

WESCH

Hi

And if you want to know 100 % exact for sure, jack the car up so that both rear wheels are off the ground. Put the gear in Neutral and turn the drive shaft by hand. Mark the wheels and once, ea wheel has done 1 full turn, remember the turns the drive shaft had to turn.
This will be the exact ratio.
3,7 turns = 3.70 rear end, 4.1 turn = 4.11 ratio etc.

The best way to assure . All others are some guessing.

Rgds. Günther

lakebumm

I AGREE!!!!!!!!!!

400hp427vette

Didnt Jack it up yet. I just drove it today. At 70mph tach reads 3000rpm anyone have basic ideal of what ratio this might be?

bj1k

Please keep your eyes on the road while doing this or your tach might go to zero real fast.

wfo76

Sounds like a 3.36 or possibly 3.55 depending on tire size. If you have the M21, the 3.36 was standard but you could also order the 3.55 or 3.70.