Since the manual transmission doesn't have the second overdrive gear and causes the engine to run higher RPMs on the freeway, I was wondering what kind of real world impact to expect on mileage.

With my 07 regular cab longbox 2500HD Silverado Classic with an LBZ/Allison all bone stock I've been getting around 18 - 20 mpg on the freeway, travelling at around 70 mph. My RPMs normally sit at around 1800 rpm with the automatic, and I figure I should be at around 2000 rpm running the same speed with the ZF6.

Would I expect to see any significant drop in fuel economy by doing the swap? I have no plans to modify the truck in any other ways, just the transmission swap.

I don't know about the 6 speed allison. But my ZF truck got better fuel mileage towing than my 2 5spd allison trucks got empty. My 02 LB7 got 16 towing 10k around town and 20ish emtpy. My 04 LB7 and 05 LLY Allisons got 15 empty and 11 towing 5k. One thing to remember is the allison take alot more power to turn 50Hp IFRC s that will hurt your fuel mileage over the manual thats got less rotating drag.

One thing to remember is the allison take alot more power to turn 50Hp IFRC s that will hurt your fuel mileage over the manual thats got less rotating drag.

I heard the same thing about allison....

Driving the I-5 from oregon to cali with about 1500-2000lb in the bed. I got about 19-20. I can only imagine it'd been more if didn't have a couple boulders in my bed... :o:

Holding 75mph empty I get 22mpg highway.

the allison takes nowhere near 50hp to run. Closer to ~10hp.

On an LB7 you'll see 30-50hp less on a dyno ZF6 vs Ally. We saw that way back in 05 ;)

the allison takes nowhere near 50hp to run. Closer to ~10hp.

I've owned both trucks and just from a seat of the pants feel and my hand calculated fuel mileage the alli pulls down alot more hp than the ZF.

IIRC Dyno numbers showed 230-240rwhp on a stock Ally truck and
280-290rwhp on a stock ZF truck. These were LB7's on a chassis dyno back in 05/06 so it's been a while. Maybe the experts have found a way to correct the numbers better now :rolleyes: