GM 6.0/4sp vs. 8.1/Allison vs. 6.0/6sp

Brisk wrote:

jmp wrote:
I have a 2003 GMC with the 8.1/Allison w 4.10 ratio with 37k miles (extra cab) towing similar weight. It tows 12-13k with ease...you just use more gas on the hills! I could not imagine a 6.0l, regardless of trans, towing this amount of weight. Keep looking for the 8.1. They are out there.

That is good info to have. Thank you. Can you run in OD on the flats pretty easily?

Yes it will run on OD on the flats.

I have the 8.1 towing my signature rig at 15K pounds. I always tow in OD and have no problems. I towed with and without tow haul on. I get a little better mpg out of tow haul mode. I just went on a trip with my brother who has a smaller fifth wheel and powerstroke f250. I paid a little less for gas than he did.

Brisk wrote:

jmp wrote:
I have a 2003 GMC with the 8.1/Allison w 4.10 ratio with 37k miles (extra cab) towing similar weight. It tows 12-13k with ease...you just use more gas on the hills! I could not imagine a 6.0l, regardless of trans, towing this amount of weight. Keep looking for the 8.1. They are out there.

That is good info to have. Thank you. Can you run in OD on the flats pretty easily?

Yes, it will tow in OD on flat ground especially when empty. It will pop out of OD on moderate inclines though. If you can find an 06-07 with the Allison 6-speed, that would be ideal. It is thirsty though even when not towing but I don't care since the motor is bulletproof.

I had a 2003 1500HD with the 6.0/4speed, 3.73 rear. Great truck, wished that I had kept it. However, it was very thirsty. It would tow very good. I now have a 2010 2500HD 4x4 with the 6.0/6speed, 3.73 rear. It gets a little better gas mileage, around 16 on the interstates, 12-13 around town and 7.5-8 while towing. My current trailer weighs around 9,000lbs and is 36 feet long. I use a 12k Equalizer hitch and it tows great. It was 5 years old on 01 March and has 73,000 miles. Put new Bilstein shocks and Michelin LTX tires on it last year. I just retired and I plan on keeping this one for a long time.

jmp wrote:
I have a 2003 GMC with the 8.1/Allison w 4.10 ratio with 37k miles (extra cab) towing similar weight. It tows 12-13k with ease...you just use more gas on the hills! I could not imagine a 6.0l, regardless of trans, towing this amount of weight. Keep looking for the 8.1. They are out there.

That is good info to have. Thank you. Can you run in OD on the flats pretty easily?

I have a 2003 GMC with the 8.1/Allison w 4.10 ratio with 37k miles (extra cab) towing similar weight. It tows 12-13k with ease...you just use more gas on the hills! I could not imagine a 6.0l, regardless of trans, towing this amount of weight. Keep looking for the 8.1. They are out there.

double post

I have an 03 2500HD with the 8.1 and Allison. The Allison is the only auto trans that was installed in trucks with th 8.1. As was previously stated the Sub and Avalanche did not get the Allison because it did not fit. The rear axle in the 8.1 trucks is different than the 6.0 trucks. It has a larger ring and pinion and is the same as is used in the Dmax trucks.

I have mostly towed a 10,000lb gross toyhauler with a quad in the bed also. No trouble maintaining speed when towing even on hills.

I would also take into consideration that the 6L90E transmission has an internal controller (TCM).

Until this application I was not familiar with any component controller that wasn't passively or actively cooled. Your engine ECM likely has cooling fins, your PC has a cooling fan and an OTR tractor has an ECM which is fuel cooled. Processors love a cool environment.

The 6L90E TCM is part of the valve body and lives inside the transmission where it is 160F on a good day.

In 2012 we bought several delivery trucks with the 6.0L engine/6L90E combo, and IMHO the 6L90e does not as age nearly well as the 4L80E.

At about 250K miles the 4L80E will give you a slip code telling you the clutches are getting thin and that it's time for a rebuild.

I have had several 6L90E fail when the TCM engaged an additional solenoid (can you say trans brake?) @ road speed and immediately deposited large pieces of metal into the oil pan.

I believe this controller is compromised in such a hostile environment.

I am not saying that they all fail that soon, only that our 6L90E premature failure rate is much higher than that of the 4L80E and that the damage to the core is much more significant. When it fails it will also require a tow truck, whereas the 4L80e will usually drag it's butt home.

I would go with 4L80E or Allison, both superb choices.

I had a GM 6.0, 6-Speed, 3.73 in the past and even with the 4.10 gearing I'm not sure I would want to push the 10k mark. Now that heavily depends on where you are towing as well. Brother in law tows a 8000lb trailer up here and complains about the lack of power on almost every trip. Again we don't have much flat roads, always some up down grade and having to regain speed.

I understand that your looking for a newer, gas truck. Have you considered a 2005-2010 F350 with the 6.8 V10 with the 5-Speed transmission? It's numbers are 362HP and 457TQ, pretty comparable to the 8.1. Only reason I bring it up is I think if your looking for "newer" and towing the heavy weights that truck with the 4.30 rear end might be the better of the newer trucks for that type of abuse.