There are three differential ratios and four chain drive ratios available for the 4T60E. This gives a total of twelve possible FINAL DRIVE ratios (FDR).

See, the input shaft drives the chain, which drives the mainshaft. The chain uses two gears to do this, the "drive" gear (input shaft) and the "driven gear" (mainshaft). If these two chain gears have the same number of teeth, we call that a 1:1 chain drive ratio. However, if the tooth count from the two chain gears is different, the chain drive ratio changes. As mentioned, there are four possible chain drive ratios, achieved by changing tooth count on the drive and driven gears (BTW, the chain is the same for all ratios). The tooth count and ratio is given as follows: drive gear:driven gear/ratio...

1. 33:37/1.12:1 (underdriven)
2. 35:35/1:1 (direct drive)
3. 37:33/.89:1 (overdriven)
4. 38:32/.84:1 (overdriven)

The effect each of these ratios has on the FDR is stated in parenthesis.

Now, the differential ratios are as follows, reflected in the number of teeth on the differential sun gear:

1. 2.84 (38 teeth)
2. 3.06 (34 teeth)
3. 3.33 (30 teeth)

The most common 6/60 FDR is 3.33. Most W-bodied 3.1's and 3100's have it. The 3800 cars and the 6/60 N-bodies use a 3.06 or 2.73. The 3.4TDC/DOHC cars use 3.43. There are other exceptions and optional ratios, etc.

Now, if you have a 3.33 ratio, you have the 35:35 chain set and a 3.33 diff. If you have a 3.06, you have the 35:35 chain and 3.06 diff. Easy to see, right?

Now the other combos... a 3.43 is a 3.06 diff with the 33:37 chain, so 3.06 x 1.12 ratio = 3.4276 or 3.43. Another common ratio is the 3.18, which is a 2.84 diff with the 33:37 chain, 2.84 x 1.12 = 3.1808 or 3.18. The 2.73 is a 3.06 diff and 37:33 chain: 3.06 x .89 = @2.73.

So now that you see how it's done, let's build the 3.73!

If you have a 3.33, you already have the highest ratio diff. You need the 33:37 chain drive set. This gives 3.33 x 1.12 = 3.7296 or 3.73. If you have a 3.06, the underdrive chain set will give you a 3.43 best. You will need to change the diff to a 3.33 to get the 3.73.

NOW, the 33:37 chain gears are about 125.00/ea from GM. Lucky for you all I have been squirreling them away over the years. They can be replaced in the car, but I would highly recommend it NOT be done that way. The diff can be changed easily and one could go from a 3.06 to a 3.33 rather easily.

REMEMBER!!!! ANY change in ratio of any kind on any "-E" transmission will require custom chip reprogramming as all shift points are dependant on road speed and rpms. These two factors change any time the FDR changes.

Thanks to Jeff Ianitello of Engineered Performance for the write-up.