Wheel Dimentions Explained

September 9, 2005

What exactly do the various numbers to describe wheel/tyre size mean? Obviously 17" would describe the diameter of the alloy itself, but after that I've no idea.

I'm looking at replacing the 15"s on my gen 6 with some 17"s (with tyres), I've been looking on eBay and just need to be able to figure out what would fit my car, n what spacers I would need. Quite a few gen7 17's going for example.

September 9, 2005

There's some info here on PCD and Offset.

The other important number is the width which is reasonably self explanatory.

With regard to tyres, you need to get a similar rolling radius to that which you already have. Thus, if you increase the wheel diameter then you'll adjust the tyre's profile so that it has lower sidewalls - result = bigger wheel, wider tyres but no increase in overall diameter of the tyre.

If you look in the wheels and tyres section you'll find a link to a calculator that will help with this.

September 9, 2005

Example for tyre size:

205/45/17

205 = Width = 205mm

45 = Aspect Ratio of the width = 45%

17 = Diameter of Wheel = 17 inches

As for the nut holes you have PCD and Offset to worry about

PCD = Pitch Circle Diameter = Diameter if you imagine a circle going through the center of each stud hole. This has to be correct

Offset = the distance from the back of the centre of the wheel to the actual midpoint of the wheel. If you have positive offset it actually moves the wheel further into the car, if you have negative offset it moves the wheel further out. A negative offset would have either a very thick middle part to the alloy or it could dish in a lot. If you have to much postive offset you can buy spacers to reduce the positive offset and take the wheels further out.

Hopefully that's all correct (going from memory!) and everything you needed to know

September 9, 2005

Thanks for that, its good to understand it at last. So I've got a PCD of 100 on my gen6, with five studs, so I take it any wheel with PCD 100 that require 5 studs will physically fit (unless you get different sized studs?). So what is the offset figure for my car? What would be to much negative offset? And the spacer size I guess would be the difference between my offset and the +ve offset of the wheels I'm buying. I guess I'm looking for somthing of very similar if not the same width and diameter. I don't understand 'aspect ratio of width%' on the tyre measurement.