20" rims are good for one thing only. appearance. that's it.
i would def. get some airbags. and 17's

 

well your only going to get soo much out of the truck. I would just tow how it is. Tow in the engines operating range, As in find a gear that holds around 2500 rpms or 4500rpms up steep hills and just hold it there. Dont try giving it more gas to keep the speed up. IF you start slowing down just hold that gear and let the truck slow down. My dad is a truck driver and tells that to everyone because most people will start downshifting and ramming the gas and thats what blows up engines.

 

Same size, just 17? Right now they are P275-60R20 so go to P275-60R17?

 

Good
point but what about the 20 inch rims?

 

it would be the same width but 3 inches shorter
you would need a 275/75 r17 to be the same size.

 

I had 20's towing a 25ft 7000lb trailer up the 15 freeway called cahone pass wich is the steepest hill in socal. I would run it 4500 rpm the whole way, getting 3 miles per gallon but dropped to 50mph. Going from 20's to 17's will make a difference but nothing where it will be Oh my god this is amazing.

 

I'll restate that.....What would you suggest in a 17 inch rim? and Tire?

 

The 20" wheels don't hurt anything. They are no different than running 33" tires on 17" wheels. My truck tows 7500+ lbs with no trouble.
The Diablosport Predator will give you an increase in power, economy and make the transmission shift better.
I would look into an extra transmission cooler since you do not have the factory tow package.
4.56 gears would also help both towing and empty.
Your truck with the tow package is rated to tow over 8000lbs so 6500 is not near maxed out.

 

What are the 17's actually doing for me?...Lowering my revs?...

 

I seem to slow alot in the hills......and the revs get wayyyyy up.