I'll be pulling an Inclosed 20ft trailer with mine and some buddies street bikes. I use overdrive when in the city and when driving up to about 60-65. But on the interstate going about 80 or so do I still need to use od? I figured not because of how high the rpms will be running. I have an 04 xlt 5.4 with mods

 

You're supposed to turn the OD off when towing.

 

That's what meant to say. But going 80 your easily up to like 3500rpms with it off

 

X2. Also, going 80 while towing a 20' trailer isnt very wise.

 

Okay, yeah your right. But still. 70-75 on flat interstate is it necessary to have it off?

 

Yes unless your truck is holding the OD gear. Its the constant shifting in and out of OD that kills your trans. Youre doing much much less damage by letting your engine sing along at a higher RPM than overheating your trans. When in doubt lock it out.

BTW, most trailer tires are only rated to 60-65 MPH.

 

So you know it's supposed to be off when towing...

Then you ask this question...if you know what's above you already know the answer to your question?