2007 GMC 1500 4x4 tire size reduction.

My summary: You are over about 5 of your truck's ratings when you tow.

9000 pounds loaded 5er
7000 pound tow rating

1500-2000 pound pin weight
250 pound receiver
xxx people

1500 pounds of payload

Smaller tires have lower load rating. Going from 31" tire to 29" tire reduced your effective gearing by 7%. That's not significant for your to notice and increase your already high risk of things that can break being overweight.

Save your tire fund money for a more capable truck. If you think you cannot afford one, sell the current one and buy a 2 year older 3/4 ton gasser for the same value as your current truck. If you still think you cannot afford that, sell the 5er and buy a smaller/lighter TT. The 5er is most certainly more valuable than a comparable vintage TT.

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.

4x4 will have to regear both front and back

TcTheHandyman wrote:
so what tire size or diameter would I need to get something like a 4:10 gear ratio?

Somewhere around a 25" or 24" tire. My chart does not go that low or tire dia.

My 4.6L Ford really woke up when I went from 3.55 gears to 4.10s. If you are stuck with the truck you have, why don't you consider actually regearing instead of reducing tire size to such extremes? One typical caveat of smaller tires is lower load handling capability.

TRAILER LIFE 2014 TOWING GUIDE
Axle Ratio:
Lower gearing (the numerically
higher ratios, e.g. 4.10:1, 4.56:1, as opposed
to 3.31:1, 3.42:1), typically produces greater
tow ratings because of greater torque multiplication.
There is a wide variance in the axle
ratio’s effect on the tow rating, with some
diesel-powered pickups varying only a few
hundred pounds; on others, changing from
a 3.73:1 to a 4.30:1 sometimes increases the
tow rating by 3,500 pounds. In the past, some
ratings have increased by a factor greater than
three (from 2,000 to 7,100 pounds) simply by
using a different axle ratio. Lower gearing will
also make your vehicle accelerate quicker up
to 50-55 MPH. In general, a one-step drop in
axle ratio (4.10:1 to 3.73:1) on an HD pickup
will drop the tow rating and the GCWR by a ton.
Lower gearing has an adverse effect on fuel
economy, but it’s usually not proportional to
the gain in towing ability. Under the best circumstances,
the difference between the highest
and lowest gear ratios offered (say, 3.42:1
and 4.10:1) results in a 1.5-MPG decrease in
non-towing, steady state-highway cruising.
In most normal conditions that change is less
than 1 MPG, and if you tow a relatively heavy
trailer a lot, there’s no question that the lowest
gearing is best.

TcTheHandyman wrote:
so what tire size or diameter would I need to get something like a 4:10 gear ratio?

That's a tough one. I get 25.8333"

You into low-profile tires?

A 4.8 will be shifting all day even trying to pull a popup

I changed the rear end gearing in my truck in hope of having more pulling power up hills. I was able to jump the speed up the same hill by 10MPH but my gas mileage went from 8MPG to 6MPG when towing... and when not towing the mileage went from 10 to 8MPG.

Not worth the effort.

so what tire size or diameter would I need to get something like a 4:10 gear ratio?