Tire tread question (comments welcomed)

[Crowz]

Tirerack is where I was at. I wonder if its because I have my X5 entered in as the vehicle?

[Crowz]

This is the page Im on :

https://www.tirerack.com/tires/tires...emeContact+DWS

I get this :

[X53Jay4.8is]

Originally Posted by Crowz

This is the page Im on :

https://www.tirerack.com/tires/tires...emeContact+DWS

I get this :

If you just search by size of tires you are looking for you will get a whole host of options as opposed to searching based on vehicle. Different ways to to get a better result.

[jcp240z]

I've run the Conti Extreme DWS. I'm running the Nitto Motivo right now. The Nittos aren't quite as good as the Contis for performance but they are a few hundred cheaper and I'm seeing better tread wear. That's important for since it is my daily driver and I put between 40 - 100 miles per day. If it was my weekend car and I was only "playing" with it I'd put the Contis on, but I am very happy with the Nittos.

[jdstrickland]

225/45x17

225 is the tire width in millimeters
45 is the percentage of the width, it equates to the sidewall height
17 is the rim diameter in inches

225 x .45 = 101.25 millimeters (double this number)
202.5 / 25.4 = 7.972 (converts millimeters to inches)
17 + 7.972 = 24.972 (25) inches of overall diameter.

Because your X5 is AWD, you want all 4 tires to be within a percent or two of the same overall diameter. This is not a difficult thing to do with a square set up, simply buy 4 tires that are all the same size. If the tire size goes up, the relative position of the speedometer goes down -- larger tires indicate slower speed when the actual speed is the same. My sample tire fits a 3 Series (E36/E46), your X5 gets a stock tire size of 235/65x17, which works out to 29.027 inches in diameter. Very simply, if you grow the rim diameter by an inch, you reduce the Aspect Ratio (sidewall height) by 5% (a 225/45x17 becomes a 225/40x18) to keep the same overall diameter.

[oldskewel]

Originally Posted by jdstrickland

225/45x17

225 is the tire width in millimeters
45 is the percentage of the width, it equates to the sidewall height
17 is the rim diameter in inches

225 x .45 = 101.25 millimeters (double this number)
202.5 / 25.4 = 7.972 (converts millimeters to inches)
17 + 7.972 = 24.972 (25) inches of overall diameter.

Because your X5 is AWD, you want all 4 tires to be within a percent or two of the same overall diameter. This is not a difficult thing to do with a square set up, simply buy 4 tires that are all the same size. If the tire size goes up, the relative position of the speedometer goes down -- larger tires indicate slower speed when the actual speed is the same. My sample tire fits a 3 Series (E36/E46), your X5 gets a stock tire size of 235/65x17, which works out to 29.027 inches in diameter. Very simply, if you grow the rim diameter by an inch, you reduce the Aspect Ratio (sidewall height) by 5% (a 225/45x17 becomes a 225/40x18) to keep the same overall diameter.

... and then to add in the effect of tread wear, just estimate the tread wear (typically measured in 32nds of an inch, starting at ~11/32, going down to 2/32 when you hit the wear bars), double it to account for diameter vs. radius, and subtract that number from the middle step above (7.972 inches in that example).

After doing that for any candidate tires vs. what you already have on there, what matters for the transfer case is the front-rear ratio. Your truck will have a spec on that, perhaps in units of something like revolutions per mile, meaning how many revs the front tires make per mile vs. how many for the rear ones.

And on my earlier comments about stability - on dry roads that will not be a factor since tread depth won't matter much. But the real risk would be on wet or snow/ice roads where you don't want the rear to slip out before the front.

And here are the specs on my 2001 3.0i, where I've got 19's:
285/45R19 107V rear
255/50R19 103V front
235/65R17 spare

If you run that formula through those 3 sizes, you'll see that they are almost identical in rolling diameter / circumference.

[StephenVA]

GREAT explanation and I love the little formulas!