tire size advice for sierra

[whincup]

ive seen several topics on sierra tire size, but none really answer my questions properly.
i have 215/75r15 tires on my sierra, and i want to upgrade to something like 30-31" on it. it has a 2" lift, so hoping they'll fit.

what i want to know is how they will affect the performance of the car?... such as speed, power and fuel economy both on and off the roads?

[Billjohn]

A 2" lift will allow you to bolt on 31" tires.
But your tread will rub the firewall, the inner fender lip and the spring when turning and articulating. Yoou can clearance the firwall and the fender lip, and even adjust the turning radius to keep the tires off the spring. But you may want a bit more lift to make the trail a bit easier.

To keep the great gas mileage the zook had from the factory, you need to keep running the stock 195/75/15 tires that came on it. Any change will effect (drop) that number. You can go to a larger tire and (kind of) make up for it with gearing, but you won't get a 1:1 return. You can increase the output of the motor and regain some speed, but you will lose some gas mileage. I have done both to more zooks than most are lucky enough to play with, but I can't show you two that have had the same results. Maybe that is why you haven't really gotten a straight answer that you can set in stone.

[whincup]

ah ok, thanks!
do bigger tires make a significant difference offroad?

[Billjohn]

Originally Posted by whincup

ah ok, thanks!
do bigger tires make a significant difference offroad?

They give you two things. More clearance under the vehicle and a larger footprint. The clearance part is pretty self explanetary, but the foot print is a little more. You can drop pressure a bit more to allow more rubber on the ground for traction. When going over obstacles you have more rubber to almost wrap around the rock to pull you through.

A larger foot print also makes for less ground disturbance as you go through a trail. The zook doesn't have the power of the big V-8's so you aren't tearing things up, but mathematically you are putting less pressure to the ground because it is spreading the weight over more surface.

So keeping control of the skinny pedal can also make you look better in the eyes of the enviro-nazies...
win-win.

[alternator]

Originally Posted by Billjohn

A 2" lift will allow you to bolt on 31" tires.
But your tread will rub the firewall, the inner fender lip and the spring when turning and articulating. Yoou can clearance the firwall and the fender lip, and even adjust the turning radius to keep the tires off the spring. But you may want a bit more lift to make the trail a bit easier.

Bill, as you know I have done a SPOA lift and got a set of Maxxis Bighorns, which are 12.5" wide. I just installed a set of Spidertrax spacers to keep the tires clear from the springs; the left side looks good but the right side is still barely touching the spring at full lock. So how do you adjust the turning radius to keep it clear? I don't think putting a second spacer is a good idea, is it?

[Billjohn]

Originally Posted by alternator

Bill, as you know I have done a SPOA lift and got a set of Maxxis Bighorns, which are 12.5" wide. I just installed a set of Spidertrax spacers to keep the tires clear from the springs; the left side looks good but the right side is still barely touching the spring at full lock. So how do you adjust the turning radius to keep it clear? I don't think putting a second spacer is a good idea, is it?

Take a close look at what hits at the knuckle when you are at full lock. You will see that it is one of the small bolts that hold the felt seal in place (going each way). If you add a washer behind the bolt it will move the head out and adjust the turning radius. Thin washer = small adjustment, thick washer = large adjustment.