2007 GMC 1500 4x4 tire size reduction.

TcTheHandyman wrote:
I am needing to increase my torque (towing power) on my truck to pull a 28 ft 5th wheel RV with a dry weight of 7,900 pounds.
2007 GMC Sierra SLE 4x4 4.8L V8
The factory tire size is P265/70 R17 (31" Diameter), with a gear ratio of 3.42 & I am thinking of getting smaller tires (possibly LT or XL 235/65 (29" diameter))to effectively change the gear ratio.
I was wanting any info you can offer on this.

I already have the truck and the 5th wheel RV, changing them out is not an option I can afford.

That ain't gonna work no matter what you do to the truck. You are looking at over 1400lb of pin weight once loaded. It will put you down on the tires. and it is not going to tow it down the road easily, and well, forget going up a hill. And there ain't no where in SW NC that is flat.
You seriously need to look for a lighter trailer. You need to be in the under 4000lb dry range. Sorry to rain on your parade. Every one else is being nice. But setting a 8000lb, probably 10,000lb loaded, 5er on a 1500 series, 4.8 truck, is just not smart. And will be really dangerous IF you can get it to move.