4spd vs 6spd Allison question.

If I remember correctly, the 8.1L could be ordered with a 4L85-E HydraMatic 4 speed standard or the optional 5/6 Speed Allison. Most of the time it was ordered with the Allison. The overall O/D ratio in the 4 speed and the 5 speed Allison are the same at .75/1. The Allison is stronger and has an extra gear in the middle. The 6 speed Allison has an advantage with the second O/D at .67/1. The gas version of the Allison has a different final O/D ratio, the diesel version has a .65/1 final O/D.

I don't know that it would help your gas mileage to gain extra gears unless you are constantly having to shift gears. GM sets them up to run at 65mph with the best torque/gas mileage combo on the GMT-800 series trucks. They try to hit the sweet spot of the two. The older 5 speed allison diesel trucks hit their peak torque at 1800rpms right at 65mph at the stock tire size. The newer GMT-900 and K2XX trucks with the duramax make peak torque at 1600rpms, mine makes it to 70mph at 1600rpms with the stock tire size. This should apply to the gas motors as well.

Find a better reason to justify a newer truck.

the 8.1 in a pickup either came with a 5 speed allison or a 6 speed allison. sae as the diesel.

the 6 speed may give you a very slight increase in mileage. 1-5 are the same gears, 6th is something like .61:1 while 5th is .7:1

my 2015.5 duramax gives me a couple mpg better than the 04.5 with the 5 speed did running empty on the highway. how much is transmission is anyones guess.

towing, I found absolutely no difference in mileage.

My Chevy Silverado 2500 HD is a 2006, has an 8.1 and a 6-speed Allison. I think that was the last or next-to-the-last year this particular combo was offered. It is a great truck made into a great little RV. 70K miles so far, no glitches. I try not to obsess over the MPG. This vehicle is for vacation and fun, not my daily driver.

You have an '02 pick-up with the 8.1L and a 4 speed? I didn't think that existed. I thought all 8.1L trucks had the 5 speed Allison. My '01 did.

Didn't the 8.1 in the pickups also come with the Allison? I think for 2002 it was a five speed?

The 8.1L in the SUV came with the 4L80/85E tranny...but I didn't think the pick ups did.

Is a diesel out of the question? 6 speed Allison...
Somewhat better mileage, lower RPM's