4spd vs 6spd Allison question.
If I can find 2007 Chev/GMC 3500 with the 8.1 I would like to think about picking one up. I drive my 2002 by the tach, 65 mph=2100 rpm. That seems to be the sweet spot for best mileage. I know I am...
Forum Discussion
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.
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.
