Originally Posted by SWHouston

Greetings MountainLady, welcome to the Forum.

I agree with Sabr's very informative post, but if I may, add a little deatil...

Drive Trains are pretty tough, and to say that increasing a Tire size (over recommended) would put more strain on them is technically true, but rarely more than they can handle.

When setting up any vehicle, the Manufacturer may offer several options on Rim/Tire Size, BUT, the one factor that they will attempt to be consistent with, is the "Diameter of the Tire". So, what they do is vary the Diameter of the Rim AND the Proportion of the Sidewall (that's the 20R through 75R number), to maintain the overall Tire Diameter. The Proportion or Height of the Sidewall to the Tread Width, is also referred to as "Profile".

Keeping this measurement within a given range as Sabr has pointed out, will allow compliance with several systems on a Vehicle. With the onset of Computer Technology on our Vehicles, though not absolutely necessary, it is wise to abide by the recommendations.

Let me give a brief example...
Here are some sizes of tires which are recommended for your vehicle.
265-75R-16 is 31.6" in diameter*.
265-70R-17 is 31.6" in diameter*.
285-50R-20 is 31.3" in diameter*.

Though the 20" is .3" smaller, that is close enough not to majorallly effect the Speedometer Reading, or other calculations the on-board Computer would make with the set reference of 31.6". You can see how the Rim Diameter, Tread Width and Profile, are manipulated to maintain the overall Tire Diameter.

Now, why would/do they have variations in this ?

Well HA, there used to be some very solid reasons for this, but, now a days I think it's more an issue of how "COOL" it looks

Anyway, if they were trying to make a Family/Luxury model, they would choose a tall Profile like a 75R. That puts more distance and cushion between the Rim and the Road, allowing for a softer ride.

IF they were attempting to create a Sports Feel to the vehicle, they would reduce the Profile like to a 50R or less, to take the sway out of the Sidewall, and increase the "control" the the driver feels/has from that.

Now, there's one other important issue, when you're replacing the tires on a given Rim. Hopefully, the person who bought the Rims, did so with good information about the requirements of the Vehicle.

Let's say you have 20" Rims, and they are 9" wide.
It's important to keep the Width of the Tire Tread, the same as the Width as the Rim. Keeping the Sidewalls of the Tires straight up and down, makes for the best Performance, Control and Tread Wear.

So, in this case, you would select the Tread Width according to the Rim Width, the Mounting Size according to the Rim Diameter, and then allow the Proportion of the Sidewall to "float", to achieve the overall recommended Diameter of the Tire.

Hope this helps,
Have a good Day !
S.W.

* = There may be some slight difference in dimensions, between different Manufacturers Tires.

Originally Posted by SabrToothSqrl

the GMT900 requires 17" wheels to clear the brakes. 16s will not work.

Originally Posted by SWHouston

rPtW,

Clearances are certainly important, and something you should verify.

I'm surprised that you aren't considering your 20's for street use, and the 16's (or what will fit) for Snow.