I just bought a set of 03' Mercedes SL55 Wheels that are 18" but slightly different width. Since the SL55 is a RWD car, the rear tire is about and inch wider than the front. I put the wheels on today and they look awesome! (who knew Mercedes and Audi use the same bolt hole and offset?) Anyway.. I was flipping through the user guide and it warns you about using different size tires front to back with different 'rolling radius' because you could damage the Quattro. My question is: can someone please give me an accurate description of 'Rolling Radius'? The tires are only about 5 mm different in diameter, but perhaps there is something else I should know. I don't want to F up the Quattro.

 

Let me start by giving you the difintions of the numbers given on the sidewall of the tire.
Example-P100/50R15

The P stands for passenger car tire. A truck tire would begin with T.
100 is the width of the tire in mm. In this case the tire is 100mm wide.
The 50 is the aspect ratio. This indicates how large the sidewall of the tire is as a percentage of the width. So in this case a sidewall percentage of 50% of the 100mm width would indicate a sidewall measurement of 50mm. That number must be doubled when a diameter mesuerement is used because there is sidewall on both sides of the rim.
The 15 stands for the diameter of the rim in inches. Kinda stupid to have measurements in two different standards huh.

So to get the diameter you would take the 15 inches and add 2 times the sidewall dimension(because there are sidewalls on both the top and the bottom of the tire.

You can also use the tire calculator at http://www.thirdgen.org/newdesign/te...ulations.shtml to figure out exactly what diameter your tire wheel combo is.

 

I have always been told in AWD vehicles never run different size tires. Right up to if you get a flat in a tire with a set of tires less than 25% tread less, just replace all of them. It is not healthy to mix match

 

Thanks Dankhound, I did know that but its good to put it here for everyone to see.

I'm still at a loss for the definition of 'Rolling Radius', but the circumferential difference between the two are 10mm. I also got some good feedback (from others and from driving the car) that the handling becomes sloppy and the understeer is accentuated. This is due to the rear track being almost 2" wider than the front. Awesome for a RWD car.. not so for a AWD apparanently (Though Lambos with AWD use this setup?) Basically, I'm not digging the way the car handles so I'm putting the 19s back on. It could also just be my car ( I have a leak in a power steering hose) but who knows.. don't want to tempt fate with the tranny.

Its a damn shame though, because the car looks downright wicked with the fat tires and sleek wheels. If someone else wants to put them on their car, shoot me a PM and I'll make you a good deal. If I don't hear from anyone, I'll list them on the classifieds page on our site and MBs.

To recap, the wheels are 18's w/ 5x112 lug and the tires are Pirelli Pzeros with almost full tread.