Settle an argument for me, please!

[ejc1214]

My buddy and I have been arguing over this for a few weeks and I keep telling him that he is wrong about this. He says that I should not drive with the tow-haul on all the time because it hurts my MPGs and I am saying that it does not make a difference once at cruising speeds as if I take it out of tow-haul my rpms do not drop and stay steady at about 2400rpms at about 62mph. Even when I was towing with my Duramaxes I just put it on tow-haul mode and forgot it. I never saw a difference in MPGs. I just like the way it holds the gear a little longer getting going and the help it gives me with the grade breaking even on the highway.
So, who is right?

[Old-Biscuit]

When not towing, using TH mode will drop MPG only due to longer length of time/higher rpms in each gear before going into OD.

Enough to notice......slightly

I can get better MPG when not using TH
BUT I like the TH mode for everyday driving......less up shifting, better downshifting.
I even drive with the aftermarket exhaust brake on also.

[Daveinet]

In theory it would hurt the mileage, but not by enough to measure. Typically it will increase the pump pressure, so running higher pressure is wasting energy, but so many other factors effect mileage much more, so it would be difficult to detect.

[oldkarphart]

I hope you bet some $.$$, cause you're right!:W

[ScottG]

It's not going to hurt your mileage any significant amount and it will make life on the trans much easier.
Not using TH is penny-foolish since trans are far more costly than the insignificant fuel savings.

[J-Rooster]

I tend to side with you, but if there is poor road conditions (rain etc.) that could change the mpg due to more road friction.

[I_am_still_wayn]

It depends on some vehicles and it appears yours does not hurt mpg once in high gear. Some tow/haul modes are programmed to lock out the over drive final gear, but it appears yours does not. However, most tow/haul modes change the shift points to keep in lower gears longer which would reduce mpg's. The question has to be asked, "Why do you want to use tow/haul mode when not towing?"

[KD4UPL]

IF it hurts MPG it would be so small as to be nearly undetectable.
At cruising speed the torque converter would be locked whether in T/H or not. The RPMs would be the same. The shift points wouldn't matter because it's not shifting.
When stopping, starting, accelerating, etc. T/H off may give the mentioned nearly undetectable increase but that's not the point. T/H mode is the proper mode according to the designer of the vehicle for the situation. It provides better acceleration/deceleration and less mechanical wear on the vehicle. I wouldn't sacrifice vehicle control and longevity for .1 MPG.