Forum

Tow/haul probably should be used, and try not to think about the gas mileage.    The thing you want to avoid is having the the transmission 'hunt' for gears… you know, when it's under some load and you see the RPMs climb about 500 but it hasn't actually shifted.  That is transmission slippage and it builds up heat fast.  A little of that when not towing is ok, but when towing you tend to get a lot of it.  Tow/haul mode probably will lock out overdrive and reduce that slippage.

So even if it shoots your gas mileage some, that's better than a new tranny!

Mike Magee said:

<  Tow/haul mode probably will lock out overdrive and reduce that slippage.>

One function of Tow/Haul is to reduce heat by locking the torque converter earlier in the gear changing sequence. IOW the engine is hard-connected to the tranny when underway.  Under Tow/Haul, the torque converted is active only when stopped.  You should feel a firmer shift between first and second gears. 

With Tow/Haul off, you will observe an rpm increase on a climb while at the same speed.  That is the torque converter at work.  If the tranny is slipping while in gear in Tow/Haul, you have a tranny problem.

HTH;

John