Duramax vs Power Stroke Fuel Economy Tow Test and Drag Race

Cummins12V98 wrote:

ShinerBock wrote:

Cummins12V98 wrote:
"I think the 3.42 rear end on the GM might have had an effect on mpg here especially if they let the truck do all the shifting."

But But, I thought the magical 10 speed was supposed to keep the engine in the perfect rpm???

This is not due to it being a 10-speed but rather the trans tuning. Shifting manually or having a shorter final gear would alleviate this. To be fair, I had to do the same in my 68RFE when it had stock trans tuning and so do my friends and family who have Aisin's so Ram's stock trans tuning is not perfect either.

I guess my 3.42 68RFE and 4.10 AISIN are the exception as they both have shifted and held gears as they should from solo to hauling 29-25k combined. Maybe cuz I ran the hell out of each right out of the box and ALWAYS run TH.

It is not a factor of holding gears or capability. My 68RFE and the Aisin 3.42's can easily hold 6th and the computer will be happy to keep you there towing at 1,500 rpm. However, this may not put you at the best rpm for efficiency and you may be using more fuel and boost to compensate for the lack of gearing just like how you would have to put more force on a shorter lever than you would with a longer one.

Both my friends/family who have Aisins and my 68RFE get better fuel mileage locking out 6th to keep the truck within the 1,800-2,100 rpm that Cummins recommends for the most efficient towing with the 6.7L. If you have a 4.10 gear, then locking out 6th may not be needed to get the best efficiency towing most loads because it will put you in these rpm ranges without it.