Have a BB 69 with M-20. Rear tires are 26.6 diameter, (P245 x 60 x 15). After driving it for years with a 3.08, 12 bolt, I bought a Motive 3.42 R & P and installation kit for a guy here to put it in Monday.
After going to a cruise-in last night, I got to wondering if I'd be happy with the gear change. The car is just a street car, no dragging, burn outs, etc, but I like to "feel it" every now and then. Not even a posi.
My biggest dislikes is clutching it some to get rolling and the longness between gears. The positive is, it's really easy to get along with cruising and on the highway. I'm not constantly changing gears.
Gas mileage isn't an issue but would probably be using the upper RPM range more with the 3.42's.
I was thinking the 3.42 would be a good in between gear but at this point I could use some input from you guys with experience with both. I realize it's my decision ultimately but you guys always make me think.
It has a pretty good cam and I attribute that to some of the "getting moving" issue. However, I had a 69 Olds W-31 with a 3.91 gears and the factory cam was touchy taking off with dispite the gear.
Thanks.
Thanks.

I don't think that you'll mind it on the highway at all with 3.42's. For 6 years I drove an old Vette with the same tire height you have on the highway, and I had 3.73's and it was fine. ;)

I would go with 3.31, with a posi unit, and you will have best of both performance,and driving ability. My son changed his 3.31 to 3.73, and I think he wishes he had stayed with the 3.31.Car runs hotter, and the performance is not that much hotter. He put 15 inch wheels on it and that made it a little better driver.

My son changed his 3.31 to 3.73, and I think he wishes he had stayed with the 3.31.Car runs hotter, and the performance is not that much hotter. .Don't know why it would run "hotter"....that souds like he has another issue going on there, such as the wrong initial ignition timing, or even an inadequare cooling system (more likely the former). But if the "performance isn't that much hotter" as you've said going from 3.31 gears to 3.73's, then what makes you think that going from 3.08's to 3.31's would even make this person's car performance anymore "hotter"? I don't think that going from a 3.08 gear ratio to a 3.31 ratio is even worth the trouble nor the expense. He wouldn't even feel any acceleration gain on the street. Wouldn't even be worth it to perform such a miniscule change in my opinion. And my opinion on this is based on the gear change I did from 3.55 gears to 4.10's, and from 4.10's to 4.56's. So I think that he's even going to seriously consider 3.31 gears, then he might as well just stay with his 3.08's. There's hardly any difference between the two.

3.31s with an Eaton 3 Series Posi is the way to go. I have this setup in my 64 Chevelle (327, M20 4 Speed). You will not regret it.

An M20 with 3.08 gears is a good set up, I would stay with that.
Ron

Have a BB 69 with M-20. Rear tires are 26.6 diameter, (P245 x 60 x 15). After driving it for years with a 3.08, 12 bolt, I bought a Motive 3.42 R & P and installation kit for a guy here to put it in Monday.
After going to a cruise-in last night, I got to wondering if I'd be happy with the gear change. The car is just a street car, no dragging, burn outs, etc, but I like to "feel it" every now and then. Not even a posi.
My biggest dislikes is clutching it some to get rolling and the longness between gears. The positive is, it's really easy to get along with cruising and on the highway. I'm not constantly changing gears.
Gas mileage isn't an issue but would probably be using the upper RPM range more with the 3.42's.
I was thinking the 3.42 would be a good in between gear but at this point I could use some input from you guys with experience with both. I realize it's my decision ultimately but you guys always make me think.
It has a pretty good cam and I attribute that to some of the "getting moving" issue. However, I had a 69 Olds W-31 with a 3.91 gears and the factory cam was touchy taking off with dispite the gear.
Thanks.
Thanks.

You WILL feel the difference going from 3.08 to 3.42 exspecially with the M20's 1st gear. I say go for it. It will make rowing gears much funner.:D

I vote 3.08's, I just went to 3.31 and I have to say as far a street use and no racing, the 3.08 is a much better gear. IMO.

Hmmm...3.08 is a 10-bolt gear ratio.

http://www.chevellestuff.com/qd/generic_rear_axle_info.htm

Do you have a 10-bolt rear end?

You can play with gear ratios, tire size, and RPMs at a given speed at http://www.chevellestuff.com/tools/rpm_calculator.htm

Hmmm...3.08 is a 10-bolt gear ratio.

http://www.chevellestuff.com/qd/generic_rear_axle_info.htm

Do you have a 10-bolt rear end?

You can play with gear ratios, tire size, and RPMs at a given speed at http://www.chevellestuff.com/tools/rpm_calculator.htm

I've heard this before...that there's not a 3.08 for a Chevy 12 bolt. Not sure where this comes from. I just put a Richmond 3.08 gear in my 12 bolt diff last year and I counted the teeth myself. 40 on the ring and 13 on the pinion. 40/13=3.0769 Maybe its the rounding that throws people off. 3.0769 rounds to 3.08.

I ran a 3.08 setup in my 66 for years. not bad for freeway cruising but for fun around town and a occasional trip to the track It just didn't have the fun factor I wanted :noway:.

Switched to a 3.42 trutrac setup over the winter :D. All I will say is it woke my car up and I am a happy chevelle owner.

TOG

I have a '65 Malibu SS with what I figure is the original 10 bolt non-posi rear: Going buy the 'very scientific' jack up the car and count the driveshaft spins method, It looks to be a 3.31 or 3.36 gear. I coun't find any ID number on the axle, but I'll look again today. From the sounds of it, is the 3.36 with the 10 both and the 3.36 with the 12 bolt originally? Just trying to get a handle on what I have without really unbolting everything in my driveway... In any case, I'm happy withe the gear for my street/driver use.

Sorry to hijack and thanks for the basic info. :disco:

10 Bolt Ratios are 3.08 and 3.36.
12 Bolt Ratios are 3.07 and 3.31.

Thanks... I'm sure that means I have the 3.36.

3.42 is a good gear- better around town and not too much on the highway.
if you think it's a little too much rpm, then you could always go with a taller tire down the road to kind of split the difference between the 3.08's and 3.42's.

How much $$ does it cost for a do it yourself swap and what are some good manufacturers? Trying to get a handle on my setup. I'm pretty sure I have a 3.36 rear and I'm running a TH350. Just about sure I like the gear and would rather run a either a 200 4r or a 700-r4. Again, sorry to hijack and thanks...

I would purchase a Motive or U.S. Gear 4.11 Ring and Pinion and you will have the best of both worlds with a 200 4R. Get a Timken Master Kit. Also, if you do not have a Posi, I would recommend installing an Eaton.
Check out this gear calculator and you will see what I am talking about:

http://www.richmondgear.com/101032.html

The 200 4R has a .67 Overdrive.

3.42 is a good gear- better around town and not too much on the highway.
if you think it's a little too much rpm, then you could always go with a taller tire down the road to kind of split the difference between the 3.08's and 3.42's.
Since 3.31s are not available new, how about a Motive (G888) 3.23 Ring and Pinion?

Hmmm...3.08 is a 10-bolt gear ratio.

[/URL]


Might have been.....but not in todays world.
http://www.ecklerscamaro.com/product.asp?pf_id=721811&dept_id=1863

This is a link to the 69 Rear Differential Code List to avoid any confusion:

http://www.chevelles.com/years/69/69rearcodes.html

Since 3.31s are not available new, how about a Motive (G888) 3.23 Ring and Pinion?

I was just going to mention that. The 3:31 ratio is not reproduced so you would have to source out a good used set.

Might have been.....but not in todays world.
http://www.ecklerscamaro.com/product.asp?pf_id=721811&dept_id=1863
Camaro Ring & Pinion Gear Set, 3.08, 12-Bolt Differential, For Cars With 3-Series Case, 1970-1981. Question concerned a 69 Chevelle which I still doubt ever had a 3.08 gear ratio. Not a Camaro guy but I thought Camaros went to a 10-bolt in 1971 and 70 was the last year of a 12-bolt in the Camaro; could be wrong though.

I have a 3:55 with a M-20, which has a 2.52 ist gear ratio as apposed to a 2.22 in the M-21 and have 14" tires with a 27" diamter. Seems like I am shifting quite a bit. Works out to 2653 rpm @ 60 mph. I have a set of original 3.07's and 3.31's and am also thinking about a change.

I have a 3.55 with a M-20, which has a 2.52 1st gear ratio as opposed to a 2.22 in the M-21 and have 14" tires with a 27" diamter. Seems like I am shifting quite a bit. Works out to 2653 rpm @ 60 mph. I have a set of original 3.07's and 3.31's and am also thinking about a change.I'm sure it's a typo but M21 (M22) clcose ratio transmissions have 2.20 1st gear.

So Dale, what your saying is that a set of 3.08 gears will not fit in a 69 chevelle 12 bolt housing?

I don't know what all this stuff about 3.08 ring & pinion gear ratios not being available is. What ratios were available as original equipment isn't even of any relevence here anyway, since even if he figures out what the car originally came with, that doesn't necessarily mean that the original gears are even still in the car, nor even that the original rear end housing is still in the car for that matter. And as far as what is available in the aftermarket for Chevelles, here's a typical example below of 3.08 gears being sold by Summit Racing on their website for a GM rear end.....

http://www.summitracing.com/search/Part-Type/Ring-and-Pinion-Gears/Brand/Motive-Gear/Axle-Model/GM-8-5-in/Ring-and-Pinion-Ratio/3-08-1/?keyword=Ring


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Part Number: MGR-GM10-308
Estimated Ship Date: 8/7/2009


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Thanks BillyGman, that was the point I was trying to make.

Thanks BillyGman, that was the point I was trying to make.Yes Rick, I noticed that, and decided to lend you a hand for the sake of avoiding any further confusion for the original poster of this thread. :yes:

With all due respect to the original post, several threads suggest the owner doesn't know what the ratio is and is searching...

http://www.chevelles.com/forums/showthread.php?t=283029 - here one poster suggests 3.31 based on tire size, rpm and speed.

Less than 2-hrs later another thread started ...

http://www.chevelles.com/forums/showthread.php?t=283041 - 3.08 is suggested here based on the same input.

Both suggestions are based on a 26.6" tire, 2200-2250 RPMs, and a 5 mph spread in speed of 55 mph to 60 mph - and that's a pretty good spread in speed in relation to RPMs.

A 215x70x14 tire (closest I could get calculated to 26.7") at 2200 RPM and 55 mph calculates to 2132 RPM with 3.08 at 55 mph and 2326 at 60 mph. A 3.31 gear calculates to 2292 at 55 mph and 2500 at 60 mph. This, of course, assumes the tachometer and speedometer readings are 100% accurate. FWIW, a more probably 3.07 gear would calculate to 2125 at 55 mph and 2319 at 60 mph - such a small differerence that a tachometer wouldn't register <10 RPM difference.

Anyone know how long the 3.08 gear set has been available for GM 12-bolt rear ends? Poster suggests he's been driving the car for some time and it would seem unlikely that a past owner would have substituted 3.08 gears for the factory 3.07 setup.

But back to the original question of going with 3.08 or 3.42, having had one of those pesky original 3.07 rears (guess I'm the only one that ever had 'em - everyone has 3.08s), it's great for highway but the 3.42 set would add less than a hundred RPMs at 60 mph over the 3.31 (2583 vs. 2500). Even driving 75 mph, the 3.42 calculates to 3229 RPM, three hundred more than the 3.07/3.08 at 2900.

It doesn't matter what ratio his car originally came with. He would be wasting his time trying to find out. And if the rear is a posi rear, I stand behind what I told him in post#5 of the thread below which is the second link that you've listed in yourt post. :yes:

http://www.chevelles.com/forums/showthread.php?t=283041

I agree that math, in this case, isn't something I'd use since (1) in-car tachs are not accurate enough (maybe 100 RPM if one is lucky) and (2) speedometers certainly aren't accurate and given the 5 MPH spread.

I don't know why Positraction would make a difference if the driveshaft were being turned by hand and comparing that to 1 revolution of a rear tire. I would also question why it being a 12-bolt would probably mean it had Positraction; only 13% of 68 Chevelles that were possible to have Positraction did so - 464,669 possible but only 60,283 were so ordered meaning 87% of those possible did not.

I never heard of turning the DRIVE SHAFT by hand as you've mentioned. I think that would be the hard way to do it rather than simply turning the tire. But if there's any advantage to doing it that way that I'm not aware of, then please state it.One board member on here has claimed that it isn't easy using the method I mentioned to find the rear end gear ratio IF the rear end isn't posi. I'm very skeptical about that claim, but I was giving that theory the benefit of the doubt since I've never tried to use that method with open rears (non-posi) myself. As far as your comment about how many 12 bolt rears are posi and how many aren't, it doesn't make much difference if his 12 bolt rear IS a posi rear. So until he chimes in and mentions if he has a posi rear or not, then it doesn't even make any sense for us to argue about that since it wasn't even a main point of my post. The main point of that post of mine in the other thread that you are commenting on happens to be that using the method I described to find the ring & pinion gear ratio would be a whole lot more reliable than trying to find out what ratio the car originally came with 40 years ago (like you seemed to be caught up with earlier in this thread) or than trying to use the tire height and speedometer to attempt to determine the gear ratio as others in that other thread were suggesting that he do.

Finding Rear End Ratio without removing the Rear Cover and Rear Jacked Up:

With a Posi you will see that as you turn the drive shaft the rear wheels turn in the same direction and you will not be able to stop one wheel without either of them stopping. Just note the # Shaft spins to one rotation of the Hub to determine Rear Ratio.

With a Limited Slip you will see that as you turn the drive shaft the rear wheels turn in opposite directions. You have to hold one wheel from spinning while turning the drive shaft and the opposite wheel will turn. Right now I can't remember but: I believe you have to ensure the Hub make 2 rotations when holding one wheel and note the number of times the shaft turns to determine the rear ratio. Just try it you see what I mean.

I used that method for years and was right on when determining ratio's and never had to remove a Rear Cover.

Im trying to figure this one out myself.

Can someone tell me if this site makes sense in trying it with a 12 bolt?
I want to say yes but would like other members opinions.

Gear Ratio Tip - Video

www.metacafe.com/watch/507848/gear_ratio_tip

Sorry about posting that video twice. I was trying to get the link only in there.

I was just going to mention that. The 3:31 ratio is not reproduced so you would have to source out a good used set.

I keep seeing this posted and I keep linking to Toms differentials who shows a 3.31 gearset on his site.

Maybe I need to tell Toms diffs that 3.31's arent made?

I had 3.42's in a '70 camaro, It was good in town and not to terrible on the highway you'll be happy with the 3.42.

Yes a typo mistake on the M21.

I have 27" diameter tires on 14" with a 3.55 and @ 60mph in 4th gear the engine turns about 2653 rpms.

With 3.31 @ 60 mph it should turn about 2473 rpms. about a 180 rpm drop.

And with 3.07 @ 60 mph it should turn about 2294 rpms. another 179 rpm drop.

I think the car would still be fun to drive which a change to the 3.31; but unsure of a drop all the way to the 3.07's.

Maybe some physics guy could give us the different rates of acceleration.

I run a 3.42 in 67ss. It is just right, I did however switch to a 700r4 last year - icing on the cake.....

well with an overdrive trans that's a different story. I had a daily driver with an O/D trans, and I installed 4.56 gears in the rear, and driving on the highway was just fine for me. In overdrive at 70 MPH, it only reved at 2,700 RPM. Nothing wrong with that as far as I'm concerned. And I took a 13.5 hour 850 mile drive with it to go see a friend with those gears in the rear, and with the .71 overdrive, it was just fine on the highway with a 28" tire height, long tube headers, and Magnaflow wide open glass pack type mufflers.

Im trying to figure this one out myself.

Can someone tell me if this site makes sense in trying it with a 12 bolt?
I want to say yes but would like other members opinions.

Gear Ratio Tip - Video (http://www.metacafe.com/watch/507848/gear_ratio_tip/)

www.metacafe.com/watch/507848/gear_ratio_tip (http://www.metacafe.com/watch/507848/gear_ratio_tip)

Yep, that's correct. :yes:

I ran 3.42's in my BBO 442 convert, 4spd and loved that set up with the M20's 1st gear:D I also ran 3.90's in my other BBO 442 HT 4spd now that was a kick-ass combo with the m20 and some help from Joe Mondello. I think this is a great thread because of all the opinions and experienced guys on this forum, HELL we never even thought about an ODtranny conversion back then. It was either a th350 or a th400 or a glide. The #38 post above^^ gives about a 400rpm difference when cruising on the highway with 3.55 -3.31-3.07.
.IMHO if you are limited to using a m20 or m21-22 for whatever reason I would go with the gear that performs the best of both worlds 3.55-3.73. If you love the performance around town thats hard to beat. If you think you are turning to many R's on the highway SLOW DOWN 5mph. Most people that see a chevelle on a highway want to take a good look anyway. You like top end than go TKO or ODtranny.

I am running 3.07 in my car. Tires are 235-60-14. At 70 mph my KK shows I'm turning 2600 maybe a little on the plus side.

The only issue is city driving around 30 mph, in fourth gear, it kind of lumbers and third gear is almost a little too much. But on the highway it wind until the cows come home :thumbsup:


Rocky

I do not think I am wrong??? My 71 307-350T Nomad wagon had a 3:08 -10 bolt. When I built the 12 bolt I removed the OEM 3:31 and I installed a (OEM ) 3:07 Ring & Pinion. I dont think it was a 3:08 ?
Bob:confused:

Just went back and look at the booklet I have from GM with the 66 chevelle specifications.

It shows that on the L35 SS the standard rear was a 3.31 and if you ordered a 4 spd you generally got the M-20.

Thus the gear reduction was: 8.34, 6.22,4.83, and 3.31 respectively for 1st, 2nd, 3rd, and 4th.

If you got the L34 SS the standard rear was the 3.73 and if you ordered a 4 spd, you generally got the M-21.

Thus the gear reduction was: 8.21,6.11,4.77,and 3.73 respectively.

I have the L34, m20 and a 3.55 my gear reduction would be: 8.95,6.67,5.18 and 3.55 respectively.

These #'s put it between a m21 with a 3.73 and m21 with 4.10's except for 4th gear. Car has good acceleration and good all around driving response. Thought maybe going back to a 3.31 would be a good compromise and still give me a gear reduction that is more then what you had with a L34 , m-21,3.73 combination.

My thinking is what was good for a L35 should be even better for a L34. Opinions?