Rear end gear ratio??
#1
Melting Slicks
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St. Jude Donor '09
Rear end gear ratio??
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?
#2
Instructor
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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.
#4
Burning Brakes
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2022 C2 of the Year Finalist - Modified
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
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
#5
Safety Car
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
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
#6
Melting Slicks
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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.
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.
#8
Drifting
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.
#12
Burning Brakes
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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.
#13
Burning Brakes
#14
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.
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.
#16
Melting Slicks
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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
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
#17
Burning Brakes
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2022 C2 of the Year Finalist - Modified
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
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
I AGREE!!!!!!!!!!
#19
Le Mans Master
#20
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.