Is 3.42 the new 4.56?

[Kyle401]

I decided to trade tow vehicles this fall and noticed something interesting while reviewing overall gear ratios of new transmission/final drive combinations. Gears 1-5 of the new 6 speed GM's with 3.42 have very similar ratios to gears 1-4 of the 4 speed with 4.56 gears. However, 6th gear with 3.42 offers an overall ratio near the old 4 speed with 3.23's.

6L80 with 3.42
1st - 13.772
2nd - 8.805
3rd - 5.239
4th - 3.940
5th - 2.914
6th - 2.281

4L65 with 4.56
1st - 13.949
2nd - 7.410
3rd - 4.560
4th - 3.174 (4.56)
4th - 2.248 (3.23)

I was on the fence about whether to put a newer 6-speed on my must have list of features, but this tipped the scales.

I have attached a file with potential tow vehicles that were evaluated in case it may be of some use to forum members. Note that there are some idealized assumptions made in the calculations. Since the same assumptions are made for all vehicles, the numbers may be used for comparison only.

We wound up with an '07 Yukon XL Denali in Silver Birch Metallic. Looking forward to a great year of camping.

[SStar]

Very interesting info, What package did you get in your Denali? My TV is an 07 Yukon XL 2500 with 4:11 rearend, 4 speed trans. So far I have been very satisfied with everything but the gas mileage, average 10 to 11 mpg towing if I stay around 60. Really do like the closed vehicle over a pickup, especially 7 passenger seating. I think you made a good choice on your Denali.

[Kyle401]

We got a 1500 with the 6.2L and 6-speed with 3.42 axles. I am expecting that my towing mileage will be similar to yours @ 10-12 mpg. We were getting 12 mpg with our 04 Silverado 5.3L with 3.73 axles.

[DKB_SATX]

The more "speeds" in the transmission, the greater the possible total ratio spread. I'm a little sorry that I cheaped out and bought an older truck with a 4-speed 2 years ago when I bought my first modern truck, though it does its job well. If I were replacing the truck 6 speeds would be the minimum. (Dodge has 8 these days, and that transmission behind the V6 diesel available now in the Grand Cherokee and Q3 on the Ram 1500 has great potential.)

[Kosm1o]

I have towed our 28' Safari with two different Silverados that were exactly the same except for the tranny. The mileage is better on the 6 speed by about 1mpg. We get 12 - 13 at about 62- 65 mph. Getting up to speed is maybe slightly better with the 6 speed.

[dznf0g]

Do your math (I'm too lazy right now) with a 3.73 and the 4 speed. Other than the hole launch in first, I think you'll find 2 - 4 closer to the 3.42 with the 6 speed.

[Kyle401]

Not sure if this is what you are asking for, but it's pretty easy to compare using the spreadsheet attached to the first post.

3.73 with 4L65
1st - 11.410
2nd - 6.061
3rd - 3.730
4th - 2.596

With the six speed and 3.42 1st is 21% lower, 2nd is 33% lower, 3rd is 40% lower and 4th is 52% lower compared to 1-4 with 3.73 and the 4 speed.

Comparatively, with the six speed and 3.42 1st is 1% higher, 2nd is 9% lower, 3rd is 15% lower and 4th is 24% lower compared to 1-4 with 4.56 and the 4 speed. Comparing 5th in the 6 speed to 4th with the 4.56 yields an overall ratio only 8% higher.

When I first started looking at newer tow vehicles, I assumed that the prevalent use of higher final drive ratios (numerically lower) was due to ever increasing fuel economy standards and performance would suffer. When I crunched the numbers, I realized that this is one instance where we really are getting the best of both worlds. Lower bottom end ratios for excellent towing and higher 6th gear OD for better unloaded fuel economy.

[dznf0g]

Sorry, I didn't even see the spreadsheet!

[Kyle401]

No problem. I was really surprised by the numbers myself. I was trading a 3.73 4 speed and set out to find 3.73 or 4.10. I didn't believe my dealer when he told me that the 3.42 six speed would perform better than a 4.10 4 speed. The numbers proved him right.

[dznf0g]

I knew it would be better than a 4 speed 3.73, but I figured a 4.10 4 speed would be better except for 6th for fuel economy. Guess it always pays to do the math.

[scamp]

I have a 2012 silverado 4x4 with 5.3liter v8. and 6 speed trans and 3.42 rear end and I'm towing a 19 foot airstream and the fuel economy while towing is 19mpg at 60 mph the truck even goes into four cylinder mode while towing! Lots of power

[jgerardi]

Great info. Thanks
Joe Gerardi