Tried to see what gear ratio my car was today, crawled under and took some "pics"

[Brandon T.]

This is on my 68 which was built in late 67 car is dated D07.

It's a 427/390hp l36 with a 4sp, not sure if the 4sp is m20 or m21 because in 68 they didn't put the code indentifier on the end of the transmission number to tell if it's a m20 or m21..

here are pics of the rear end I can see the 67 date code which makes sense for my car so it's the original rear end just not sure on the ratio which I know theres other ways to tell but wanted to see what it was stamped with see if you can make it out I sure can't.

Some of you guys that know what to look for might can make it out I took a lot of pictures some show the part that I can't make out better than others, if someone could make that out and I know for sure what came in the car that would make my day

the last few pics show it best

[Wayne M.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

Brandon -- in spite of the numerous pics, I can't tell what the ratio code is. There's been some pretty severe wire-wheel ? brushing in there that may have made it worse.

Here is a pic of a 1967 diff that was donated to me (now used as a door stop ) that I took to show you where to look, spacing-wise for the first 2 ratio letters on the driver side. This one was badly corroded on the flange of interest, and as the carrier was empty, I used a smooth flat file to "unearth" the code AU (3.36 big block; one of the sweetest all-round ratios known to Corvette). The full stamping, of which you can make out the 5_17 in the photo, is to add _67 at the end. The casting number (which is usually well preserved under the leakage from the driver side yoke) is D_12_7 (about a month separation before assembly).

You may want to check this second date, to correlate with what seems to my eyes to be 12_12_67; can't make out anything to the left of that.

As for M20 vs M21, is the car driveable ? Then take it up to 30mph indicated exactly in 1st gear, then do the same in 4th and report back the RPM's in each case. This will also give us an idea of what the rear ratio is, if the speedo and tach are not way out of calibration.

[Jack P.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

Brandon -- in spite of the numerous pics, I can't tell what the ratio code is. There's been some pretty severe wire-wheel ? brushing in there that may have made it worse.

Here is a pic of a 1967 diff that was donated to me (now used as a door stop ) that I took to show you where to look, spacing-wise for the first 2 ratio letters on the driver side. This one was badly corroded on the flange of interest, and as the carrier was empty, I used a smooth flat file to "unearth" the code AU (3.36 big block; one of the sweetest all-round ratios known to Corvette). The full stamping, of which you can make out the 5_17 in the photo, is to add _67 at the end. The casting number (which is usually well preserved under the leakage from the driver side yoke) is D_12_7 (about a month separation before assembly).

You may want to check this second date, to correlate with what seems to my eyes to be 12_12_67; can't make out anything to the left of that.

As for M20 vs M21, is the car driveable ? Then take it up to 30mph indicated exactly in 1st gear, then do the same in 4th and report back the RPM's in each case. This will also give us an idea of what the rear ratio is, if the speedo and tach are not way out of calibration.

A very easy way to tell gear ratio. Jack up rear of car. Mark drive shaft with chalk. Mark rear tire with chalk. Spin wheel. If wheel is spun once. The drive shaft will turn 3.70 times for a 3.70 rear. 3 1/2 times for 3.55

3 times for a 3.8 . Easy and accurate.

Jack

[Jack P.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

Brandon -- in spite of the numerous pics, I can't tell what the ratio code is. There's been some pretty severe wire-wheel ? brushing in there that may have made it worse.

Here is a pic of a 1967 diff that was donated to me (now used as a door stop ) that I took to show you where to look, spacing-wise for the first 2 ratio letters on the driver side. This one was badly corroded on the flange of interest, and as the carrier was empty, I used a smooth flat file to "unearth" the code AU (3.36 big block; one of the sweetest all-round ratios known to Corvette). The full stamping, of which you can make out the 5_17 in the photo, is to add _67 at the end. The casting number (which is usually well preserved under the leakage from the driver side yoke) is D_12_7 (about a month separation before assembly).

You may want to check this second date, to correlate with what seems to my eyes to be 12_12_67; can't make out anything to the left of that.

As for M20 vs M21, is the car driveable ? Then take it up to 30mph indicated exactly in 1st gear, then do the same in 4th and report back the RPM's in each case. This will also give us an idea of what the rear ratio is, if the speedo and tach are not way out of calibration.

A very easy way to tell gear ratio. Jack up rear of car. Mark drive shaft with chalk. Mark rear tire with chalk. Put car in neutral, Spin wheel. If wheel is spun once. The drive shaft will turn 3.70 times for a 3.70 rear. 3 1/2 times for 3.55

3 times for a 3.8 . Easy and accurate.

Jack

[Kenneth T.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

Brandon,

As stated, it is difficult to tell in the photos. I am posting the codes and maybe if you look closely you may be able to make it out. The code, I believe, is stamped on the driver side of the diff, with the date more to the pass side.

427 \ 3.08 AT \ 3.36 AU \ 3.55 AZ \ 3.70 FA \ 4.11 FB .

Ken

[Edward B.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

Follow Jack's advice to determine your ratio. Remember, the internals may have been changed regardless of external markings. For even better accuracy, rotate the rear wheel ten times rather than once - it makes counting driveshaft revolutions a little tedious, but in the long run makes it easier to tell what ratio gearset is installed.

[Brandon T.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

thanks guys,

from looking at the pics looks like an A something to me you can see the tip form of what looks to be an A but maybe not? can't be for sure.

since I don't have a floor jack near by its in the barn..

with my car being a 427 4sp lets assume m20 what were the gear ratio options?

if it hadn't of rained all day I would have taken it down the road like the guy above said and looked at my rpm's

btw what rpm's at 30mph in 1st would I be looking for to tell what ratio

[Wayne M.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

...with my car being a 427 4sp lets assume m20 what were the gear ratio options?

btw what rpm's at 30mph in 1st would I be looking for to tell what ratio

Brandon --- here's from a neat little table in the 1965 Dealer Showroom Brochure. If you've got near stock diameter rear tires, the numbers should be close enough:

Miles per Hour per 1000 engine RPM in 4th gear
---------------------------------------------
3.08 = 26.6
3.36 = 23.1
3.55 = 22.2
3.70 = 21.3
4.11 = 19.2
4.56 = 17.0

So at 3000 rpm in 4th, if you indicate 66.6 mph on the speedo, then you have 3.55 gears (etc...)

Now, put it in 1st gear and take it back up to 3000 rpm. If you have a close ratio, speedo would indicate 66.6 divided by 2.20 = 30.3 mph. If you have a wide ratio, the speedo will read 66.6 divided by 2.54 (or is it 2.52 ?) = 26_1/2 mph.

[Brandon T.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

cool deal! i'm about to go try thato ut it's dried up some here now..one of my floor jacks is way off in the barn so this will work better than dragging that up to the house.

thanks

[Wayne M.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

....with my car being a 427 4sp lets assume m20 what were the gear ratio options? ....

I forgot you asked about rear gear options with 427 M20 (wide ratio). According to the 1968 Power Teams chart in your handy NCRS pocket guide, for both the 390 hp version (L36) and the 400 hp version (L68), you could order 3.08 (standard), or 3.36 (economy). Same choice with or without C60 air conditioning.

So if original, the codes would be AT (3.08) or AU (3.36). Also shown for 3.08 are two other codes AV and AW. Perhaps some C3 specialist could explain the need for those.

[Brandon T.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

back from a little joy ride I have some numbers..I didn't grab the printout of what you posted when I left out the door arrgh.

but here are some numbers I think can tell us what ratio it is.

at 30 mph in 2nd as far as my dad could tell driving it was reading 1900 rpm

at 30 mph in 3rd gear it was reading 1500 rpm


then to see if it has the m20 or m21 transmission I had him put it at 40 mph in 3rd gear, then shift to 4th and we noted the rpm change...at 40 mph in 3rd it was reading 1900 rpm and when he shifted to 4th it was reading 1500 rpm at 40 mph.

that help? not sure if the gauges etc are perfectly in cal or whatnot..

the engine idles at 600-700rpm which sounds about right so I assume the tach is right.

[Wayne M.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

....to see if it has the m20 or m21 transmission I had him put it at 40 mph in 3rd gear, then shift to 4th and we noted the rpm change...at 40 mph in 3rd it was reading 1900 rpm and when he shifted to 4th it was reading 1500 rpm at 40 mph.....

Amazing what a simple road test (and accurate instruments) can tell.

From what you've provided, you have a close ratio transmission with a 3.08 ratio differential.
[I had one of those once, with a '65 350hp small block -- lots of fun launching on the hills, but you should have no problem with a 427].

The math: At 40 mph, it indicated 1900 rpm in 3rd, and 1500 rpm in 4th at the identical speed. Your ratio 3-to-4 is 1.27, which matches exactly the 3/4 ratio in the specifications for close ratio trannys. Wide ratio would be 1.48.

Final drive -- 4th gear at 40 mph indicates 1500 rpm. So if you lugged it at 1000 rpm it should show 26.7 mph, which matches the table I quoted earlier for 3.08 ratio. Of course, assuming instruments OK (which they seem) and tire diameter close to an original F70-15 bias ply.

[Brandon T.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

so i have a close ratio transmission with a 3.08 gear rear end?

well that just sucks...a close ratio and 3.08 gear ACK.

wouldn't have thought u coulda got a m21 with a 3.08 gear? plus when you put this car in 4th it'll still pull up hills like a champ.

and I know for a fact nobody has ever changed the gears that it came with because I have talked to the original owner and the guy we bought it from said he had never changed anything.

o well, hard to get everything you want when you're talking about cars this old

[Brandon T.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

Forgot to ask with me finding out it's an m21 today instead of an m20 is that a good thing? I would think so but not sure how the m20 and m21 are viewed..

the m21 keeps the rpm's up which makes it a better performer when you're talking about performance right?

trying to look at the good I found out today instead of thinking about that 3.08 gear..lol

[Jim T.]

Re: Tried to see what gear ratio my car was today, crawled under and took some "pics"

Forgot to ask with me finding out it's an m21 today instead of an m20 is that a good thing? I would think so but not sure how the m20 and m21 are viewed..

the m21 keeps the rpm's up which makes it a better performer when you're talking about performance right?

trying to look at the good I found out today instead of thinking about that 3.08 gear..lol

Depends on what you are going to use your car for. I had a 3:08 M20 in my 64 Corvette and really liked it even in daily driving with the 327/300HP engine. Going from the west coast to east coast one summer and then back to Dallas, the 3:08 helped in fuel economy.
The M20 helps in everyday driving when leaving from a stopped position, especially if you are on a incline road.
If you are driving every day looking for a street race or drag racing at a strip all week-end the M21 may be to your liking.
4th gear in both are the same.