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Discussion Starter · #1 ·
I have a 68 Camaro it had an automatic took out the automatic and put am-21 4 speed the car is high gear now is not easy to take off in first high geared through 2nd and 30rd, the rear end is a 12 bolt posi we turned the back wheels and came up like 345 gears (not right form our test the right gear ratio). Am sure we have to match the transmission to the rear end anyone has some advice to solve this problem? 8x15 rims
 

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Sounds like a problem calculating the gear ratio. The automatics of the era generally came with 2.73 or 3.08 gears. The original M-21 has a 2.20 low gear which will be difficult to leave the line without slipping the clutch although it can be mastered. The M-21 generally came with 3.73 or lower gear ratio.
The gear you want will depend on your usage - race, cruise, or highway. There are a lot of posts on selecting the right gear.
 

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Welcome to the site! I'm running a M21 behind a mild BB with 3.31 gears and stock sized tires. I get along just fine with that combo. If you have an engine that doesn't make much torque at lower rpms, I can see it being a handful to drive. Something doesn't sound right about your calculated gear ratio, though.
 

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I agree with buenymayor. Do you know any specs about you engine. That would help. What size/cubic inch, horsepower/torque?
 

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1969 Camaro Restomod
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I had a 2.20 close ratio box in a couple of my corvettes back in the day. Perfect with a 4.11 rear I thought, and I even ran a 4.56 for some track use. I'm re-doing my rear in the Camaro now, going from the 2.73 in the old 8.2 rear to a 3.42 in the new 8.5 rear, but my Super T10 has a decent 1st gear so my launches will be a lot smoother. The old 2.20 Muncies were butter smooth (at least mine were) but you might be better off looking for a trans swap and keeping the 3.42 rear if that's what it is. All depends on what you prefer for highway rpm and performance.
 

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Discussion Starter · #7 ·
View attachment 290059
I have a 68 Camaro it had an automatic took out the automatic and put am-21 4 speed the car is high gear now is not easy to take off in first high geared through 2nd and 30th Tire Wheel Automotive lighting Helmet Automotive tire
, the rear end is a 12 bolt post we turned the back wheels and came up like 345 gears (not right form our test the right gear ratio). Am sure we have to match the transmission to the rear end anyone has some advice to solve this problem? 8x15 rims
Sounds like a problem calculating the gear ratio. The automatics of the era generally came with 2.73 or 3.08 gears. The original M-21 has a 2.20 low gear which will be difficult to leave the line without slipping the clutch although it can be mastered. The M-21 generally came with a 3.73 or lower gear ratio.
The gear you want will depend on your usage - race, cruise, or highway. There are a lot of posts on selecting the right gear.
Tire Wheel Automotive lighting Helmet Automotive tire

I have a 68 Camaro it had an automatic took out the automatic and put am-21 4 speed the car is high gear now is not easy to take off in first high geared through 2nd and 30th, the rear end is a 12 bolt posi we turned the back wheels and came up like 345 gears (not right form our test the right gear ratio). Am sure we have to match the transmission to the rear end anyone has some advice to solve this problem? 8x15 rims
I do have this rear end that came on my donor car what I understand is a 12 bolt main that came with 2.29:1 to 2.79: gear gm made for highway driving is what I want, any chance this may help my problem? My car has a 400 c.i. engine Thanks for the is well appreciated Russ
 

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1968 Camaro LS3 TH400 Moser 9” DSE mini tubs
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You could look on the axle tube for the letter/number stamp. There’s no guarantee that the gearing has not been changed, but it might give you a more accurate idea.
It sounds like you desire a quicker take off , but that is easier with a 3.73:1 than a 3.08:1 ratio.
It’s a compromise,
3.73 rear gives quick take off and 3000rpm at 65mph
3.08 rear gives slower take off and 2000rpm at 65 mph
Those are arbitrary numbers but you get the idea.
 

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View attachment 290059


View attachment 290109

I do have this rear end that came on my donor car what I understand is a 12 bolt main that came with 2.29:1 to 2.79: gear gm made for highway driving is what I want, any chance this may help my problem? My car has a 400 c.i. engine Thanks for the is well appreciated Russ
Pull the cover and see exactly what you have It's only 12 bolts. Lol
 
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Discussion Starter · #10 ·
Sounds like a problem calculating the gear ratio. The automatics of the era generally came with 2.73 or 3.08 gears. The original M-21 has a 2.20 low gear which will be difficult to leave the line without slipping the clutch although it can be mastered. The M-21 generally came with 3.73 or lower gear ratio.
The gear you want will depend on your usage - race, cruise, or highway. There are a lot of posts on selecting the right gear.
Took the cover off the rear end had 3.08 gears been getting a lot advise most say 3.73 gears not racing, more like cruise and highway driving any thoughts?
 

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Took the cover off the rear end had 3.08 gears been getting a lot advise most say 3.73 gears not racing, more like cruise and highway driving any thoughts?
3:73 local only unless you have OD trans. Otherwise you and your engine will not like it for long.
 

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Do thanks have any more advice?
John's recommendation above is more mainstream if you plan to go anywhere. I stick with my 3:73 because I don't go that far and the roads I am on are usually max 60mph which puts me at 3000rpm which is kind of just above the start of the sweet spot on my engine. If I was hitting the freeway for any length of time I would have to change. But with drivers today I hate to leave the driveway. They don't care.
 

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1968 Camaro LS3 TH400 Moser 9” DSE mini tubs
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I too am geared too low for the highway. 3.70 with no OD, but I’ve had 3.08 and 3.73 and I prefer the lower for leaving.
…..90% of the “ no drivingest idiots “ in Texas , live in my county 😆🤬
 

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I too am geared too low for the highway. 3.70 with no OD, but I’ve had 3.08 and 3.73 and I prefer the lower for leaving.
…..90% of the “ no drivingest idiots “ in Texas , live in my county 😆🤬
I tend to disagree with your last statement. We have like 6M people in the Houston Galveston area, I think that equates to more idiots down here. Most are not from here. Guy that hit my truck was texting, 8k damage. Girl who hit my truck was from Oklahoma, destroyed her car, I got paint on my bumper. That is one reason the cars don't go out much except around the block. After my truck was repaired for 8K someone smacked my rear bumper. Figure now I can use it as a weapon against idiots.
 

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1968 Camaro LS3 TH400 Moser 9” DSE mini tubs
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Scott,
I think your correct in having more idiots, but I think we could have a higher ratio. It’s hard to tell…. I rode a KZ 1000 back in the 80s and the only thing you had to be watching for, was the women putting on makeup on the way into Fort Worth on 35, but let me digress and say ….I’d rather be geared a little low, and have that get gone quicker, than spare my engine a few hundred- 1500 rpm. Obviously the owners choice.
Yep I’m hard on everything from engines to women.
it just the way I’m geared 😆
 

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Scott,
I think your correct in having more idiots, but I think we could have a higher ratio. It’s hard to tell…. I rode a KZ 1000 back in the 80s and the only thing you had to be watching for, was the women putting on makeup on the way into Fort Worth on 35, but let me digress and say ….I’d rather be geared a little low, and have that get gone quicker, than spare my engine a few hundred- 1500 rpm. Obviously the owners choice.
Yep I’m hard on everything from engines to women.
it just the way I’m geared 😆
Sounds like good gearing. MC is another animal. I am scared to ride my 900 around here. But back a few years ago when we had a Hurricane scare after Katrina and they asked for evacuation there was complete gridlock. I could ride the bike anywhere while everyone else just sat there in the heat. I don't think it even rained. So much for forecasts, only job you can screw up every day and go back to work tomorrow.
 

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68 Base Camaro 355 Offy crossram Richmond Super Street close ratio 5 spd ivy gold 92K SoCal car
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I have a 68 Camaro it had an automatic took out the automatic and put am-21 4 speed the car is high gear now is not easy to take off in first high geared through 2nd and 30rd, the rear end is a 12 bolt posi we turned the back wheels and came up like 345 gears (not right form our test the right gear ratio). Am sure we have to match the transmission to the rear end anyone has some advice to solve this problem? 8x15 rims
I think if you swapped the M21 for an M20 wide ratio you would be very happy with your likely 3.42 or 3.55s thst you have. The 400 should have enough torque down low launching it wouldnt be a problem.

This is what I run with a 355 sb. But a 383 or 400 I would probably opt for M20 w/3.31 or 3.55 on a street car that also sees some highway use.

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It doesnt shift as smooth as a Muncie, nothing does, but it can be close if you change out the synchros that come in them.
 

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Discussion Starter · #20 ·
hey, thanks for all the help lots of good advice thinking of going with a m-20 with 3:73 not sure am all in yet, Not sure I can find one. I do have a 10 rear end not sure what gears are in it came with a 327 auto Thanks Russ
 
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