Originally Posted by yokev

Ideally you want to return your gear ratio to what it was stock.
This is achieved the easiest way by changing out your ring and pinion located in your differential(s). If you have a 2wd, you only have ONE ring and pinion set which is located inside your rear differential.
If you have 4wd, then you have TWO differentials(front and rear) that would need to have different ring and pinions installed
Apart from the difference in tire/wheel weight(and sometimes LARGER tires and wheels weigh LESS than the smaller stock ones), installing the proper gear ratio will return the acceleration you're used to, possibly keep your truck from 'Lugging', and take the stress from the taller tires off your ring and pinion and axles. Sorry, but it doesn't do anything for the brakes
To calculate the correct gearing, I need to know your stock tire size and gear ratio.
For now I'll use my stock tire size, and what I think was your stock gear ratio:
275/65/18
3.55:1
If this is your stock tire size and gear ratio, you would need 3.88 gearing to return to your EXACT stock ratio. I don't think they make 3.88s, but they DO make 3.90s, which is close enough. Don't worry about not being spot-on, as it's more common than not to have to install a ring and pinion ratio that is slightly different than the actual size you calculated.
IIRC the next step UP(taller gearing) would be 3.73's.
It's definitely possible they make gearing for our trucks that fall in between 3.88 and 3.73's, I just don't remember any so you'll wanna check.
I have 3.73:1 gearing and recently installed 33" tires. I'd need to install 3.84's to return my truck to its stock gear ratio, but since it's really close to 3.73, and I don't even think they make 'em in that size, I wouldn't need to change.
This week I'm going to be changing UP to 34" tires, which would require a 3.96 gear ratio. As I mentioned before, they DO make 3.90s, but in this case I'm still only a little more than 6% taller than stock, so I'm not gonna worry about it at this point.
My personal opinion is 1/2 and 3/4 ton trucks having gas motors need shorter gearing with 35" or taller tires, and 3/4 or 1-ton trucks having diesel engines need shorter gearing with 37" or taller tires.
A couple exceptions would be GM 3/4 tons with the 'old' 8.1L torque moster, as GM equipped these trucks with the Allison trans and 'Diesel' running gear, and any 3/4 or 1-ton that came stock with 4.11s or shorter

Originally Posted by Grandpa's Grocery Getter

Thanks Kev..
I think for now Im safe depands on the weight of the tires I get...and im looking for lightest i can go. I know the 34" I just put on are only 5lbs per tire more then the 31.9" I had on there.......I can spin these pretty easy I have to watch at times when i take off......

Grant

Originally Posted by yokev

18's are primarily 'E' rated 10-ply, so they're gonna be the heaviest.
The 33/18's I had weighed 75lbs each, and the 315/70/17's(34") I'm putting on are 10lbs lighter, despite being bigger.
Nitto(and I'm sure others) has a 'P'assenger 285/70/17 that only weighs 45lbs or so, yet are rated for 2200lbs each.

Originally Posted by NASSTY

I doubt the gears will get torn up but the truck will be pretty sluggish on the lowend with 35's and 3.31's. Once you get momentum going it won't be too bad. Your highway MPGs should still be faily good with 3.31's but your city MPGs will probably suffer. Lower gears or a tuner will help regain some low end grunt. If you don't tow anything heavy maybe you can live with it being a little sluggish on take offs?

Due to the cost of a front and rear gear swap you may be better off sticking with the 3.55's. I don't know how much it would cost for a gear swap in your area but depending on parts and labor costs it could run you as much as $1200-$1500. I have a friend that sets up rear ends all the time so I can get it done for the cost of parts and beer. 3.55's wouldn't be too bad for regular driving and should tow ok as long as your trailer isn't too heavy. If you do decide to make the swap you should go with 4.10's. 3.73's wouldn't make enough difference from 3.55's to justify the cost.