[Cmyskill]

Switching out the rear-end may be one of my first real mods, but I think the PCM will need some reprogramming and that scares me a bit. My 4th Gen Transam came Stock with a 2.73 in the rear-end and a 4L60e and when I upgrade the rear to a 3.73 ring and pinion the car really came alive. It was as if the car instantly added 50-75 lbs. of torque to the wheels. I tried the 4:10s but that was way to much of a gear as the RPM's were ridiculously high at 60 to 70MPH on the highway.

This time if I do the swap to 3.73's, I will buy an assembled unit from GM and skip out on someone installing the gears as they Always seamed to hum if the shims are off by a few thousandths. It also could have been the cheap Richmond gear set.

So What would the top speed be if we switched out to the 3.73's?

I cannot remember the math...lol If someone has the formula I would like to figure it out for each gear and see what the RPM's will be at 70 MPH in 10th gear.

Here is most of the info to figure it out...

Red line starts at 6,500K

With the stock tires that is: 305x30x20

[17CamaroZL1]

I think the rev limiter is actually 6600, but the A10 shifts around 6500, so I used 6500 for each gear.

_______Gear: 2.85___Gear: 3.73
1st: ___ 38.11mph __ 29.12mph
2nd: ___ 59.90mph __ 45.77mph
3rd: ___ 83.31mph __ 63.65mph
4th: ___ 99.51mph __ 76.03mph
5th: __ 117.83mph __ 90.04mph
6th: __ 139.93mph _ 106.92mph
7th: __ 179.11mph _ 136.86mph
8th: __ 210.72mph _ 161.01mph
9th: __ 259.58mph _ 198.34mph
10th:_ 279.86mph _ 213.84mph
R: _____ 36.78mph __ 28.10mph

[Blast]

Hi, I used revs/mile in my formula and not the circumference like this:

Revolutions/h = Revolutions/m * 60
Revs/mile = 742

Top Speed in specific gear (in mph):
Speed = Revolutions/h / ( Revolutions/mile * RearEnd * Gear )

So first gear (4.70) with a 2.85 rear end would be :
Speed = (6500 * 60) / ( 742 * 2.85 * 4.70 ) = 39.24mph

I added 4.10 rear end just for kicks.

The table then looks like this :

                         Rear End A10
Gear      Ratio     2.85    3.42    3.73    4.10
----      -----     ----    ----    ----    ----
First      4.70    39.24   32.70   29.98   27.28
Second     2.99    61.68   51.40   47.13   42.88
Third      2.15    85.78   71.48   65.54   59.63
Fourth     1.80   102.46   85.38   78.29   71.22
Fifth      1.52   121.33  101.11   92.71   84.24
Sixth      1.28   144.08  120.07  110.09  100.15
Seventh    1.00   184.42  153.69  140.91  128.20
Eighth     0.85   216.97  180.81  165.78  150.82
Ninth      0.69   267.28  222.73  204.22  185.79
Tenth      0.64   288.16  240.13  220.18  200.31
Reverse    4.87    37.87   31.56   28.93   26.32

RPM@60mph (10th) 1353.41 1624.09 1771.30 1947.01
RPM@70mph (10th) 1578.98 1894.77 2066.52 2271.51
RPM@80mph (10th) 1804.54 2165.45 2361.74 2596.01

If I change Revs/mile to 764 I get the same table as 17CamaroZL1. I'm not saying I'm doing it right, though, so please teach me if I failed.

I also added RPMs with different rear ends in tenth gear.
And now I also added a 3.42 Rear end.

// Stefan

[joelster]

In all actuality the true top-speed will be nearly the same no matter what rear gear you choose. The car doesn't make enough power to hit redline in 10th gear with 2.73's. Way too much drag. The difference will be that with 3.73's it will be in a higher gear when reaching its top speed. I would drive one with 2.73's and see how it feels. If it goes through an intersection and is already in 3rd gear I'd bet that switching to a higher rear gear would be a waste. The transmission has a steep first gear.

[TJay74]

I am pretty sure the ZL1 / A10 combo will be plenty fine off of the line and in general day to day driving. Also, the only option you will have for a OEM eLSD housing will be the other gear set the M6 car gets.

And yes, you will need the ECM/TCM reprogrammed for this upgrade. The dealer will not be able to handle that for you either, it will require custom programming thru HP Tuners, EFIlive or some other tuning suite. Then at that point you are now putting your entire powertrain warranty at risk. Changing the rear diff out will kill any warranty on it at all, have a non-OEM tune in the ECM/TCM could cause problems down the road as well if it requires the dealer to send in a ECM/TCM snapshot to GM for validation.

That is a lot of chance to take on a brand new $65k car, much less one that testing drives have shown that the A10 transmission straight out gets after it.

[KMPrenger]

I have to ask. With 10 speeds, is lowering the rear end really necessary? Won't it just make 1st gear pretty much useless at the strip? I realize with the old 4 speeds this was a very worthy upgrade, but as we add more and more gears to the auto, seems to make this mod much less useful.

Don't bash me...just a thought!

[KaBoom1701]

Old school mentality meets new school tech.....lol

[17CamaroZL1]

Originally Posted by Blast

Hi, I used revs/mile in my formula and not the circumference like this:

Revolutions/h = Revolutions/m * 60
Revs/mile = 742

Top Speed in specific gear (in mph):
Speed = Revolutions/h / ( Revolutions/mile * RearEnd * Gear )

So first gear (4.70) with a 2.85 rear end would be :
Speed = (6500 * 60) / ( 742 * 2.85 * 4.70 ) = 39.24mph

I added 4.10 rear end just for kicks.

The table then looks like this :

                         Rear End A10
Gear      Ratio     2.85    3.42    3.73    4.10
----      -----     ----    ----    ----    ----
First      4.70    39.24   32.70   29.98   27.28
Second     2.99    61.68   51.40   47.13   42.88
Third      2.15    85.78   71.48   65.54   59.63
Fourth     1.80   102.46   85.38   78.29   71.22
Fifth      1.52   121.33  101.11   92.71   84.24
Sixth      1.28   144.08  120.07  110.09  100.15
Seventh    1.00   184.42  153.69  140.91  128.20
Eighth     0.85   216.97  180.81  165.78  150.82
Ninth      0.69   267.28  222.73  204.22  185.79
Tenth      0.64   288.16  240.13  220.18  200.31
Reverse    4.87    37.87   31.56   28.93   26.32

RPM@60mph (10th) 1353.41 1624.09 1771.30 1947.01
RPM@70mph (10th) 1578.98 1894.77 2066.52 2271.51
RPM@80mph (10th) 1804.54 2165.45 2361.74 2596.01

If I change Revs/mile to 764 I get the same table as 17CamaroZL1. I'm not saying I'm doing it right, though, so please teach me if I failed.

I also added RPMs with different rear ends in tenth gear.
And now I also added a 3.42 Rear end.

// Stefan

You're right on the money. The 742 revs/mile doesn't take into account the load of the car on the tire. The 764 number does, and therefore gives more accurate real world speeds in each gear. I like your idea of adding a couple more gear options and the cruising RPM at different speeds!

Originally Posted by joelster

In all actuality the true top-speed will be nearly the same no matter what rear gear you choose. The car doesn't make enough power to hit redline in 10th gear with 2.73's. Way too much drag. The difference will be that with 3.73's it will be in a higher gear when reaching its top speed. I would drive one with 2.73's and see how it feels. If it goes through an intersection and is already in 3rd gear I'd bet that switching to a higher rear gear would be a waste. The transmission has a steep first gear.

Exactly. I suspect the top speed will be aero limited in the 190 range.

Originally Posted by KMPrenger

I have to ask. With 10 speeds, is lowering the rear end really necessary? Won't it just make 1st gear pretty much useless at the strip? I realize with the old 4 speeds this was a very worthy upgrade, but as we add more and more gears to the auto, seems to make this mod much less useful.

Don't bash me...just a thought!

I agree. Multiplying the first gear ratio with the final drive gives a really low gear that will already be difficult to hook up with all the torque of the LT4. I think going with a lower rear gear will only render 1st useless or add an additional gear change in the 0-60 and quarter mile runs. It will also give you thinner teeth on the ring and pinion which are more likely to break, but 3.73s are typically pretty tough gears, just not as strong as the 2.85 will be.

[Cmyskill]

Originally Posted by KaBoom1701

Old school mentality meets new school tech.....lol

The more I think about it you are correct. The old 4L60e was only a 4 speed tranny. So the gear change helped out but with the 10 speed it may not make that much of a difference...

Thanks for the math lesson guys....

[RockRacer]

A couple other things to keep in mind, particularly if you are going to the strip.

1. Drag radials...are you staying at the same tire height or going shorter/taller?
2. Are you trapping at the high end of a particular gear?

Ultimately, if you switch to a shorter gear, look for combos that will give you the max benefit (time in highest gear) of the extra shift.

[Blast]

Originally Posted by 17CamaroZL1

You're right on the money. The 742 revs/mile doesn't take into account the load of the car on the tire. The 764 number does, and therefore gives more accurate real world speeds in each gear. I like your idea of adding a couple more gear options and the cruising RPM at different speeds!

Thanx!

I updated my sheet with 764 instead of 742 and added some more speeds.

And yes, I agree that with 10 gears with this broad ratio spread it may be unnecessary, but only testing will prove or disprove that in the end. Extra sticky tires on a prepped surface and you may be able to hook it well enough so that a different rear end makes sense.

I also wrote the table knowing that for another reason - I am sure someone else will ask the same or think the same in the future and then they will just search and find this table or someone will point out to them that this table already exists here.

                         Rear End A10
Gear      Ratio     2.85    3.42    3.73    4.10
----      -----     ----    ----    ----    ----
First      4.70    38.11   31.76   29.12   26.49
Second     2.99    59.90   49.92   45.77   41.64
Third      2.15    83.31   69.42   63.65   57.91
Fourth     1.80    99.51   82.92   76.03   69.17
Fifth      1.52   117.84   98.20   90 04   81.91
Sixth      1.28   139.93  116.61  106.92   97.27
Seventh    1.00   179.11  149.26  136.86  124.51
Eighth     0.85   210.72  175.60  161.01  146.48
Ninth      0.69   259.58  216.32  198.34  180.44
Tenth      0.64   279.86  233.22  213.84  194.54
Reverse    4.87    36.78   30 65   28.10   25.57

RPM@60mph (10th) 1393.54 1672.24 1823.92 2004.74
RPM@65mph (10th) 1509.66 1811.60 1975.81 2171.80
RPM@70mph (10th) 1625.79 1950.95 2127.79 2338.86
RPM@75mph (10th) 1741.92 2090.30 2279.78 2505.92
RPM@80mph (10th) 1858.05 2229.66 2431.76 2672.98

// Stefan

[speedyman]

Does anyone go 179 MPH?

[Blast]

Originally Posted by speedyman

Does anyone go 179 MPH?

You mean in the quarter mile? On the street? On a track? On the autobahn? Dropping it from a plane?

If the latter I don't think it'd go much faster than ~142mph.

I think it would help to be a little specific

// Stefan

[17CamaroZL1]

Originally Posted by Blast

Thanx!

I updated my sheet with 764 instead of 742 and added some more speeds.

And yes, I agree that with 10 gears with this broad ratio spread it may be unnecessary, but only testing will prove or disprove that in the end. Extra sticky tires on a prepped surface and you may be able to hook it well enough so that a different rear end makes sense.

I also wrote the table knowing that for another reason - I am sure someone else will ask the same or think the same in the future and then they will just search and find this table or someone will point out to them that this table already exists here.

                         Rear End A10
Gear      Ratio     2.85    3.42    3.73    4.10
----      -----     ----    ----    ----    ----
First      4.70    38.11   31.76   29.12   26.49
Second     2.99    59.90   49.92   45.77   41.64
Third      2.15    83.31   69.42   63.65   57.91
Fourth     1.80    99.51   82.92   76.03   69.17
Fifth      1.52   117.84   98.20   90 04   81.91
Sixth      1.28   139.93  116.61  106.92   97.27
Seventh    1.00   179.11  149.26  136.86  124.51
Eighth     0.85   210.72  175.60  161.01  146.48
Ninth      0.69   259.58  216.32  198.34  180.44
Tenth      0.64   279.86  233.22  213.84  194.54
Reverse    4.87    36.78   30 65   28.10   25.57

RPM@60mph (10th) 1393.54 1672.24 1823.92 2004.74
RPM@65mph (10th) 1509.66 1811.60 1975.81 2171.80
RPM@70mph (10th) 1625.79 1950.95 2127.79 2338.86
RPM@75mph (10th) 1741.92 2090.30 2279.78 2505.92
RPM@80mph (10th) 1858.05 2229.66 2431.76 2672.98

// Stefan

Great work! Your chart definitely looks better than what I could produce in the chat box of the forum! This could prove very helpful for someone contemplating a rear gear change.

And you're right. You just never know what the outcome of a gear change may be until you or someone tries it. I'm curious, but not curious enough to be the first to try it. Lol!